The average value of f(x, y) over the given region (I'll call it T) is
[tex]\dfrac{\displaystyle \iint_T f(x,y) \, dA}{\displaystyle \iint_T dA}[/tex]
In other words, it's the ratio of the integral of f(x, y) over the region to the area of the region.
The set T can be expressed as
[tex]T = \left\{(x,y) \,:\, 0\le x\le9 \text{ and } 0\le y\le -\dfrac29x+x\right\}[/tex]
Since it's a triangle, the denominator is easily computed to be
[tex]\displaystyle \iint_T dA = \frac12 \times 9 \times 2 = 9[/tex]
In the numerator, we have
[tex]\displaystyle \iint_T (10x+3y) \, dA = \int_0^9 \int_0^{-2x/9+2} (10x+3y) \, dy \, dx \\\\ = \int_0^9 \left(\frac32\left(-\frac{2x}9+2\right)^2+10x\left(-\frac{2x}9+2\right)\right) \, dx = 288[/tex]
and hence the average value is 288/9 = 32.
