Based on the given image, the coordinates of the pre image are:
A (-3,4)
B (-1,4)
C (1,1)
D (-5,1)
When you do a reflection in the line y = -x ; (x,y) coordinate becomes (-y,-x)
A (-3,4) → (-(4),-(-3)) → A' (-4,3)
B (-1,4) → (-(4),-(-1)) → B' (-4,1)
C (1,1) → (-(1),-(1)) → C' (-1,-1)
D (-5,1) → (-(1),-(-5)) → D' (-1,5)
I attached what the reflection will look like over line y = -x