Writing iterated integral of a function Write an iterated integral of a function f for the region given by a triangle with vertices at point (1,1), (1,2), (3,0).

I figured that I'm supposed to first find the equations of the three lines representing the sides of the triangle.
(1,2) and (1,1) make $x=1$
(1,1) and (3,0) make $y= -(1/2)x+(3/2)$
(1,2) and (3,0) make $y= -x+3$
Is that all I need to do? In other words, is my iterated integral the following?
$$ \int_{1}^3 \int_{\frac{1}{2}x+\frac{3}2}^{-x+3} f(x,y) dydx $$
It seems too simple and I'm afraid I'm missing something.

Yep, it's as easy as that! Note that you got the slope of your second line wrong: it should be:
$$
y = \frac{-1}{2}x + \frac{3}{2}
$$
