Differentiate $f(x)$ = $(3x - 2x^2)(5 + 4x)$

The correct answer is $-24x^2 + 4x + 15$

As per Product rule,

$\dfrac{d}{dx} (f(x)g(x))$ = $f(x)g′(x) + f′(x)g(x)$

$∴$ $f ' (x)$ = $(3x - 2x^2)(0 + 4) + (5 + 4x) (3 - 4x)$
= $12x - 8x^2 + 15 + 12x - 20x - 16x^2$
= $-24x^2 + 4x + 15$