Right answer?

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

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).

Yes, you have the correct answer.

To arrive at that answer, you performed the distributive property to simplify the expression. Here's a step-by-step breakdown of the process:

1. Start with the given expression: 15(x^2+3x) + 7(x^2+3x) - 5(x^2+3x)
2. Distribute the coefficients to each term inside the parentheses: 15x^2 + 45x + 7x^2 + 21x - 5x^2 - 15x
3. Combine like terms: (15x^2 + 7x^2 - 5x^2) + (45x + 21x - 15x)
4. Simplify the coefficients: 17x^2 + 51x

So, the final answer is 17x^2 + 51x. Well done!