Let \( f \) be a function such that \( 3 f(x) + 2 f\!\left( \frac{m}{19x} \right) = 5x, \, x \neq 0 \), where \( m = \sum_{i=1}^{9} (i^2) \). Then \( f(5)-f(2) \) is equal to:
\( -9 \)
\( 9 \)
\( 36 \)
\( 18 \)