5x² + 28x + 15
[ax² + bx + c] Multiply the first term and the last term (a and c) together
5 × 15 = 75 Find the factors of 75 that add or subtract to 28 (b)
Factors of 75: 1, 3, 5, 15, 25, 75 [1 × 75, 3 × 25, 5 × 15]
3 and 25 add up to 28
Instead of 28x:
5x² + 28x + 15
You can replace it with:
5x² + 25x + 3x + 15 [25x and 3x add up to 28x]
Now factor the equation separately. Factor 5x out of (5x² + 25x), and factor 3 out of (3x + 15).
5x(x + 5) + 3(x + 5) Factor out (x + 5)
(x + 5)(5x + 3) The first option is your answer
