What should be added to

Let $x$ be added to $\frac{-7}{8}$ so as to get $\frac{5}{9}$.

$\therefore x+\frac{-7}{8}=\frac{5}{9}$

$\Rightarrow x=\frac{5}{9}-\frac{-7}{8}$

$\Rightarrow x=\frac{40}{72}-\frac{-63}{72}$

$\Rightarrow x=\frac{40-(-63)}{72}$

$\Rightarrow x=\frac{40+63}{72}=\frac{103}{72}$