The derivative is the rate of change of a function, basically represents the slope at different points. To find the derivative of the given function you can use the power rule, which means, if n is a real number, d/dx(x^n)= nx^(n-1). This is a simplification of the chain rule based on the fact that d/dx(x)=1. Anyway, this means that d/dx(x^3 + 1)= 3x^2. Here n is 3 and so it is 3*x^(3-1)= 3x^2. The derivative of x^3+1 is 3x^2.
If you are wondering what happened to the 1, for any constant C, d/dx(C)=0.
4x+6+7x+15=120 combine like terms it becomes 11x+21 =120 take away 21 from 120 , leaves you with 11x=99 divide by 11 to get x alone leaves you with x=9 :)
