I'm assuming you meant to write a^4 = 625.
If that is the case, then note how 625 = 25^2, and how a^4 is the same as (a^2)^2
So we go from this
a^4 = 625
to this
(a^2)^2 = 25^2
Apply the square root to both sides and you'll end up with: a^2 = 25
From here, apply the square root again to end up with the final answer: a = 5 or a = -5
As a check:
a^4 = (-5)^4 = (-5)*(-5)*(-5)*(-5) = 25*25 = 625
a^4 = (5)^4 = (5)*(5)*(5)*(5) = 25*25 = 625
Both values of 'a' work out