Question 652348
It's best to rearrange this to 30z^2 + 91z - 30  (Az^2 + Bz + C)

First, multiply A*C  (30*-30 = 900)
Ask "What two numbers multiply to get -900 that add to get B (91)?"
Answer: 100*-9 (trial and error)
Rewrite B using the pair you just found: 
30z^2 + 91z - 30
30z^2 + 100z - 9z - 30
Parenthesize:
(30z^2 + 100z) - (9z - 30)
Factor each parenthesis:
10z(3z + 10) - 3(3z + 10)  <<- the parenthesis will always be the same.  The first one is always right.
Rewrite: One () is the stuff on the inside once, the other () is the stuff on the outside:
(3z + 10)(10z - 3)