Does someone knows what equals 12.5

What is the value of x in the following equation. -x ^3/2 = -27

Axel follows these steps to divide 40 by 9: start with 40. 1. divide by 9 and write down the remainder. 2. write a zero after the remainder. 3. repeat steps 1 and 2 until you have a remainder of zero. examine his work, and then complete the statements below.

Factor and solve to find roots x squared -x - 90 =0