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.