#### Answer

$$-1$$

#### Work Step by Step

In order to simplify the given expression, we look for common factors in the numerator and the denominator. Next, we cancel out these common factors to simplify completely. Doing this, we find:
$$ \frac{7s^2-28t^2}{28t^2-7s^2}\\ -\frac{7s^2-28t^2}{7s^2-28t^2}\\ -1$$