<s>[INST] equation $x^2+ax+6a = 0$ has integer roots, Then integer values of $a$ is If the equation $x^2+ax+6a = 0$ has integer roots, Then integer values of $a$ is
$\bf{My\; Try}::$ Let $\bf{\alpha,\beta}$ be two roots of given equation $x^2+ax+6a = 0$
So $\bf{\alpha+\beta = -a}$ and $\bf{\alpha \cdot \beta = 6a}$ and $\bf{\alpha,\beta \in \mathbb{Z}}$
So $\bf{\alpha \cdot \beta =-6\alpha -6\beta \Rightarrow 6\alpha+\alpha \cdot \beta +6\beta +36 = 36}$
So $\bf{(\alpha+6)\cdot (\beta+6) = 36 = 6\times 6 = 9\times 4 = 18\times 2 = 36\times 1}$ and many more
But I did not understand how can i calculate for all ordered pairs.
Is there is any other method to solve it
Help Required
Thanks
 [/INST] What you have done so far is absolutely correct. However, for the sake of completeness I will repeat the beginning of your solution. Using Vieta's equations, we know that $$\alpha+\beta=-a$$ and $$\alpha\beta=6a$$
By substituting the first equation into the second, we get:
$$ \alpha\beta=-6\alpha-6\beta$$
Which simplifies to:
$$(\alpha+6)(\beta+6)=36$$
Since we are only interested in $a$ a solution $(x,y)$ would be the same as a solution $(y,x)$, hence, we assume without loss of generality that $\alpha \leq \beta$.
$$ $$ Hence, we know that $$(\alpha+6,\beta+6)=(1,36),(2,18),(3,12),(4,9),(6,6),(-36,-1),(-18,-2),(-12,-3),(-9,-4),(-6,-6)$$
Which give us our values for $\alpha$ and $\beta$:
$$(\alpha,\beta)=(-5,30),(-4,12),(-3,6),(-2,3),(0,0),(-42,-7),(-24,-8),(-18,-9),(-15,-10),(-12,-12)$$
Therefore, the possible values for $a$ are:
$$a=-25, -8, -3, -1, 0, 49, 32, 27, 25, 24$$
However, I do not think that this was your problem in solving the question. As indicated in your question, we were unsure how to calculate all ordered pairs. The key to solving problem was realizing that there were only a small number of ways to express 36 as the product of 2 integers. In fact, there were only 10 ways to do this. This is true for most small integers. When in doubt as to the feasibility of testing all cases, it can be useful to calculate the number of factors that the integer has. This can be done by breaking the number down into its prime factorization. For example, $36=2^2 \cdot 3^2$, to find the number of positive factors, we simply find the number of ways to separate $2^2 \cdot 3^2$ into 2 groups. There are 3 possibilities for 2, we can either have $2^0$ or $2^1$ or $2^2$ in the first group, each of which will produce unique factors, similarly, there are 3 possibilities for 3. Hence, the number of positive factors of $36$ is $3 \cdot 3=9$ As such, the total number of factors that $36$ has is $18$ (including negative factors). The number of ways to express 36 as the product of $2$ integers would, under normal circumstances, be half this, as each factor should have a unique other factor with which it can multiply to form $36$, (e.g. $1 \cdot 36=36$), however, 36 is a perfect square and has to repeated factors($6\cdot 6$ and $-6 \cdot -6$). Hence, the number of ways 2 express $36$ as the product of $2$ integers is $\frac{9\cdot 2 -2}{2} +2=10$, which is quite feasible to test. 
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