<s>[INST] If $3(x +7) - y(2x+9)$ is the same for all values of $x$, what number must $y$ be? If we let $x = 0$.
\begin{align*} 
3(0+7)-y(2(0)+9) \\
21-9y \\
\end{align*}
Then $9y$ should always equal $21$?
Solving for $y$ finds $\frac{7}{3}$.
But $3(x+7)-\frac{7}{3}(2(x)+9)$ does not have the same result for diffrent values of $x$.
Where am I going wrong?
 [/INST] $3 (x + 7) - y (2 x + 9)=21-9y$ for $x=0$ and
$3 (x + 7) - y (2 x + 9)=24 - 11 y$ for $x=1$
They are the same, so must be
$$21-9y=24-11y\to y=\frac32$$
</s>