To rationalize the denominator of the expression \(\frac{15}{7 \sqrt{3}}\), we will multiply both the numerator and denominator by \(\sqrt{3}\):
\[ \frac{15}{7 \sqrt{3}} \cdot \frac{\sqrt{3}}{\sqrt{3}} = \frac{15 \sqrt{3}}{7 \cdot 3} \]
Now, simplify the denominator:
\[ 7 \cdot 3 = 21 \]
Thus, the expression becomes:
\[ \frac{15 \sqrt{3}}{21} \]
Next, we can simplify this further by dividing both the numerator and denominator by their greatest common divisor, which is 3:
\[ \frac{15 \div 3}{21 \div 3} = \frac{5 \sqrt{3}}{7} \]
Therefore, the final answer is:
\[ \frac{5 \sqrt{3}}{7} \]