Answer
(a) $4(x-3)(x-3)$
(b) $x(x-1)(x+4)$
Work Step by Step
Part (a)
$4x^{2}-24x+36=$
$4(x^{2}-6x+9)=$
$4(x^{2}-3x-3x+9)=$
$4(x(x-3)-3(x-3))=$
$4(x-3)(x-3)$
Part (b)
$x^{3}+3x^{2}-4x=$
$x(x^{2}+3x-4)=$
$x(x^{2}-1x+4x-4)=$
$x(x(x-1)+4(x-1))=$
$x(x-1)(x+4)$