Factor the expression 20g3 + 24g2 – 15g – 18

First, put parenthesis around the first two numbers and the last two numbers. 
(20g³+24g²) (-15g-18)
Then, take out the greatest common factor of both parenthesis. 
4g²(5g+6)-3(5g+6)
You then separate the numbers outside the parenthesis and the numbers in the parenthesis. 
(5g+6) (4g²-3) 
Then you simplify the second set of numbers. Since the set of numbers can't be simplified, you would leave this problem as it is. I hope this makes sense.