That has no simple anti-derivative do swap the order of integrals:
First, you need to be careful about the limits of integration. The best way to do that is to draw a "picture". The outer integral is from x= 0 to so draw vertical lines on an xy- coordinate systems at those values of x. The "inner" integral is from y= x to so draw the slant line y= x and the horizontal line . You should see that the region you are to integrate over is the upper half of the square with vertices at (0, 0), , , and . Overall, y goes from to and, for each y, x goes from x= 0 to .