Question

expand then sliplify : 3z (2z +4) + 6z (2z - 5)

Answer 1

Let's expand first:
$6z^2+12z+12z^2-30z$

Simplify (combine like terms):
$18z^2-18z$
$18z(z-1)$

So, your most simplified answer is 18z(z-1) or 18z²-18z.