To find the missing numerator si that both fractions are equivalent, you have to multiply both sides of the equal sign by 72:
[tex]\begin{gathered} 72\cdot\frac{7}{8}=72\cdot\frac{?}{72} \\ 63=\text{?} \end{gathered}[/tex]The missing numerator is 63
The equivalent fractions are:
[tex]\frac{7}{8}=\frac{63}{72}[/tex]