lomas
posted by math .
Right answer?
15(x^2+3x) + 7(x^2+3x)  5(x^2+3x)
15x^2+45x+7x^2+21x + 5(x^23x)
15x^2+45x+7x^2+21x5x^215x
17x^2+51x

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).