Hey there! I'm happy to help!
The greatest common factor is the largest factor that both numbers contain. Let's look at the factors of these terms.
12x²: 1, 12, 2, 6, 3, 4, x, x
15x³: 1, 15, 3, 5, x, x ,x
We see that both of these numbers contain a 3 and 2 x's, so our GCF would be 3x².
Have a wonderful day!
