Solve the Exponential Equation by Using the Property That $b ^ { x } = b ^ { y } \text { implies } x = y \text {, for } b > 0 \text { and } b \neq 1 \text {. }$

Multiple Choice

Solve the exponential equation by using the property that $b ^ { x } = b ^ { y } \text { implies } x = y \text {, for } b > 0 \text { and } b \neq 1 \text {. }$
$6.8 ^ { x - 3 } = 6.8 ^ { 2 x + 21 }$

A) $\left\{ 6.8 ^ { - 24 } \right\}$
B) { }
C) $\{ 9 \}$
D) $\{ - 24 \}$