Right answer?

Question

15(x^2+3x) + 7(x^2+3x) - 5(x^2+3x)

15x^2+45x+7x^2+21x + 5(-x^2-3x)

15x^2+45x+7x^2+21x-5x^2-15x

17x^2+51x

Answer 1

x^2 + 3x is common to all terms. So we
factor it out:
(x^2 + 3x) (15 + 7 - 5),
(x^2 + 3x)17,
17(x^2 + 3x).

Factor your answer and it wil be identical to mine.

Alternate method:
Let (x^2 + 3x) = a,
15a + 7a - 5a = 17a,
17(x^2 + 3x).