To solve the equation \( \frac{3}{7} = \frac{x}{49} \), we can use cross-multiplication:
\[ 3 \cdot 49 = 7 \cdot x \]
Calculating the left side:
\[ 3 \cdot 49 = 147 \]
Now, equating the two sides gives:
\[ 147 = 7x \]
Now, divide both sides by 7 to solve for \( x \):
\[ x = \frac{147}{7} = 21 \]
Therefore, the correct answer is:
21