Answer
$log_{7}\frac{3}{4}$
Work Step by Step
The quotient property of logarithms tells us that $log_{b}\frac{x}{y}=log_{b}x-log_{b}y$ (where x, y, and, b are positive real numbers and $b\ne1$).
$log_{7}15-log_{7}20=log_{7}\frac{15}{20}=log_{7}\frac{3}{4}$