Answer
$\frac{9x^{4}-4x^{2}}{21}$
Work Step by Step
Step 1: $\frac{3x^{4}}{7}-\frac{4x^{2}}{21}$
Step 2: $\frac{3x^{4}\times3}{7\times3}-\frac{4x^{2}\times1}{21\times1}$
Step 3: $\frac{9x^{4}}{21}-\frac{4x^{2}}{21}$
Step 4: $\frac{9x^{4}-4x^{2}}{21}$
Step 5: No further simplification possible, so the answer is $\frac{9x^{4}-4x^{2}}{21}$
