Let \( f:\mathbb{N} \to \mathbb{N} \) be a function such that \( f(m+n) = f(m) + f(n) \) for every \( m,n \in \mathbb{N} \). If \( f(6) = 18 \), then \( f(2)\cdot f(3) \) is equal to:
\( 6 \)
\( 54 \)
\( 18 \)
\( 36 \)