This problem involves a linear expression in terms of $x$, and we are given two values of $x$ and the corresponding values of the expression. We can use these two pairs of values to find the coefficients $a$ and $b$. Let's start with the first pair: when $x = 2$, the expression is $15$. Plugging in $x = 2$ into the expression, we get $a + 2b = 15$. This is a linear equation in $a$ and $b$, and we need another one to solve for both variables. The second pair gives us another equation: when $x = 5$, the expression is $3$. Plugging in $x = 5$ into the expression, we get $a + 5b = 3$. Now we have a system of two linear equations with two variables. We can use elimination or substitution to solve for $a$ and $b$. Let's use elimination: subtracting the first equation from the second, we get $3b = -12$. Dividing both sides by $3$, we get $b = -4$. Now that we have $b$, we can plug it back into either equation to find $a$. Let's use the first one: $a + 2(-4) = 15$. Simplifying, we get $a - 8 = 15$. Adding $8$ to both sides, we get $a = 23$. So we have found $a = 23$ and $b = -4$. The problem asks us to calculate $a + b$. Adding $a$ and $b$, we get $23 + (-4) = 19$. 
An algebraic expression of the form $a + bx$ has the value of $15$ when $x = 2$ and the value of $3$ when $x = 5$. Calculate $a + b$.