All you integrate is f(x,y). Just need to worry about limits. Sketch the area first. You should find it clear that x ranges from 0 to 9, but what about x? Well it ranges from y=0 on the lower boundary to y=x on the upper boundary, so integrate from 0 to x butinsidethe x integral (so dydx). Or integrate y from 0 to 9 and x from y to 1 dxdy. ^_^