Originally Posted by Red X

take the derivative of the individual pieces.

dy/dx(72x^2 - 4x^3)= dy/dx(72x^2)-dy/dx(4x^3)

=144x - 12x^2 => 12x(12-x)

If you don't want to open up the parenthesis then you can use the rule:

f*g' + g*f' = (fg)'

so:

f= x^2, f'=2x
g=72-4x, g'=-4

you get:

x^2*(-4)+(72-4x)(2x) = 12x(12-x)