$$\frac{8+3}{8}-\left(-\frac{7}{8}\right)$$
Add $8$ and $3$ to get $11$.$$\frac{11}{8}-\left(-\frac{7}{8}\right)$$
The opposite of $-\frac{7}{8}$ is $\frac{7}{8}$.$$\frac{11}{8}+\frac{7}{8}$$
Since $\frac{11}{8}$ and $\frac{7}{8}$ have the same denominator, add them by adding their numerators.$$\frac{11+7}{8}$$
Add $11$ and $7$ to get $18$.$$\frac{18}{8}$$
Reduce the fraction $\frac{18}{8}$ to lowest terms by extracting and canceling out $2$.$$\frac{9}{4}$$