Answer
$\{ 9.095\}$
Work Step by Step
Step 1. Rewrite the equation as $3(1.4)^x=64$ or $(1.4)^x=\frac{64}{3}$
Step 2. Take natural logarithm on both sides to get $x\ ln(1.4)=ln\frac{64}{3}$, thus $x=\frac{ln\frac{64}{3}}{ ln(1.4)}\approx9.095$
Step 3. Write answer in set form $\{ 9.095\}$