It's a binomial times a binomial, so you FOIL
First multiply the First numbers
3z*z = 3z(^2)
Then multiply Outers:
3z* -4 = -12z
Nest, Inners:
9*z = 9z
Finally, the Lasts:
-4*9 = -36
Then, throw it all together in descending order, starting with the variable of highest degree to lowest:
3z^2 - 12z + 9z - 36
Last of all, simplify coefficients with like terms and you're good to go:
3z^2 - 3z - 36
Hope this helps!