The answers are:
P ' = (-4,-7)
Q ' = (-8,-7)
R ' = (-3,3)
------------------------------------------------------------------------------------
All you do is swap the x and y coordinates of the given points P, Q and R
The general rule is (x,y) ---> (y,x)
So that's why something like P = (-7,-4) becomes P ' = (-4,-7) when we reflect over the line y = x. The other points are handled the same way.