Answer
$-\frac{3}{4}$
Work Step by Step
We factor the expressions in order to cancel out common factors in the numerator and the denominator:
Step 1: $\frac{3x-21}{28-4x}$
Step 2: $\frac{3(x-7)}{-4(x-7)}$
Step 3: $\frac{3}{-4}$
Step 4: $-\frac{3}{4}$
