Parse a basic expression and determine its derivative, returning the new value as a string.

So simple expressions with positive values like 3x^4 will return 12x^3

and x^3 will return 3x^2

All numbers will be integers, a missing coefficient is in front of the x is 1. The exponent value will always be greater than 1.

All expressions will be in similar format

4x^7

3x^5

x^9 etc