Algebra1

5i

It is 3 ^5(root5)i

BUT at high school level it is fair to say that the square root of a negative number does not exist so there is no answer.

Maybe you mean
what is the cube root of -125 in which case the answer is -5 because -5*-5*-5 = -125

there is a cube root button on your calculator or you can use.
Also a cube root is a power of 1/3
so
(-125)^(1/3) should also give you the correct answer.