(a).
-15ab-6ab^2
= -(15ab+6ab^2)
= -ab(15+6b)
= -3ab(5+2b)
(b).
6x(2y-5)-3(5-2y)
= 6x(2y-5)+3(2y-5)
= (6x+3)(2y-5)
= 3(2x+1)(2y-5)
Tip: If you want to check your answer for (a), you can always simplify it back and see whether it is same as the question.
Hope this helps : )
