Problem -

Find all values of x=c so that the tangent line to the graph of

f(x) = 4x^3(14x^2 +7x-21)^2 at (c,f(c)) will be horizontal.

the the solution the book gives me is 4x^3(14x^2+7x-21)(28x+7) + (12x^2)(14x^2+7x-21)^2 - this part i know how to get to. then 196x^2(x-1)(14x-9)(3+2x)(x+1) - which i don't know how they got to.

can someone please help explain how the book get the last part of the solution? thank you!