Let \( f : \mathbb{R} \to \mathbb{R} \) be a continuous function such that \( f(3x) - f(x) = x \). If \( f(8) = 7 \), then \( f(14) \) is equal to:
\( 4 \)
\( 10 \)
\( 11 \)
\( 16 \)
