Average temperature of a metal plate

Suppose the temperature of a flat metal sheet is given by $T(x,y) = 5xy+x^2$ . Find the average temperature on the plate in the diamond region where $| 2x+y | \leq 4$ and $| 2x-y | \leq 4$ .

If your answer can be expressed as $\dfrac{a}{b}$ , where $a$ and $b$ are coprime positive integers, find $a+b$ .