# Simple Factoring Problem

## Homework Statement

$$28s^2 + 8st - 20t^2$$

The answer in the back of the book is 4(7s - 5t)(s + t)

2. The attempt at a solution

$$4(7s^2 + st + st - 5t^2)$$
$$4(7s^2 + st + st - 20t^2)$$
$$4(s(7s + t) + t(s - 20t))$$

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In your first attempt at the solution, why not combine the second and third terms inside the (), it might make things more obvious.

Is there some trick to this that I am missing?

$$4(7s^2 + 2st - 5t^2)$$

cristo
Staff Emeritus