Elementary Algebra

$-\frac{3}{4}$
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: $-\frac{3}{4}$