:-

Assuming that should be shown as :-

5 ( x - 5 )² = 500

( x - 5 )² = 100

x - 5 = ± 10

x. = - 5 , 15

-5(x-5)^2=500. Dividing by -5 each side gives:

(x-5)² = -100.  Taking the root each side gives

(x-5) = ±√(-100) = ±√100 √(-1) = ±10 * i.  Add 5 each side gives your two answers. Done!

For -5(x-5)^2=500 the answer is: 

x=5+10i, 5-10i