625 as 5^4 power giving you:

5d^4 = (5^4)(d^16)

then divide both sides by 5d^4 (dividing subtracts exponents)

1 = 5^3 d^12

divide by 5^3 which is 125
1/125 = d^12 then take the twelfth root

d = +/- twelfth root (1/125) or fourth root (1/5)
which is (1/5) ^ 1/4
or 5^ -1/4