Answer
-$\frac{1}{3}$
Work Step by Step
$\frac{20}{21}$ -$\frac{10}{21}$ -$\frac{17}{21}$
Subtract the numerator. Keep the common denominator
=$\frac{20-10-17}{21}$ =
=$\frac{-7}{21}$ =-$\frac{7\times1}{7\times3}$
= -$\frac{1}{3}$