Hey there I need some help with this problem

Let be the region 0 <(or equal to) y <(or equal to) x

0 <(or equal to) x <(or equal to) 1

Evaluate

double integral over D of (x+y) dxdy

by making the change of variables x=u+v, and y=u-v

Check your answers by evaluating the iterated integral directly by using an iterated integral.

Please explain each step you did, so I can understand how to do this problem thanks.