The derivative of x^3 can be found using the power rule, which can be applied to polynomials of the form ax^n. When the coefficient of x is larger than one, the two numbers are multiplied together.
The power rule states:
d/dx[ax^n]=nax^(n-1) where a,n are constants
So for the derivative of x^3, since the coefficient is 1, then the number does not change. The coefficient is 3 because 1 times 3=3, and the exponent is reduced by 1. Hence, d/dx [x^3]=3x^2.