Let's take a look at each term separately.
15a^2b^2:
15 has factors 1, 3, 5, 15
a x a
b x b
-24ab
-24 has factors 1, 2, 3, 4, 6, 8, 12, 24
a
b
Now, we can see what each of these terms has in common. Both have a 3 in their factor lists, as well as one a and one b.
Therefore, the greatest common factor is 3ab.
Hope this helps!! :)