## Elementary Algebra

To evaluate the expression, we substitute the values $x=-7.8$ and $y=8.4$ into the expression, $5x-7y$ =$5(-7.8)-7(8.4)$ =$5(-\frac{78}{10})-7(\frac{84}{10})$ =$(-\frac{390}{10})-(\frac{588}{10})$ =$\frac{-390-588}{10}$ =$\frac{-978}{10}$ =$-97.8$