Nonhomogeneous Equations & Resonance · Hard problem

A $2$-kg mass hangs from a spring with spring constant $18$ N/m and no damping. Starting from rest at equilibrium, it is driven by a force $F(t)=5\cos 3t$ N (exactly at resonance). Find the velocity of the mass, in m/s, the first time ($t>0$) it returns to the equilibrium position $x=0$.