The angle of the pendulum is given by θ = θmcos(ωt + φ), where ω = 3.24 rad/s. If at t = 0, θ = 1 rad and dθ/dt = -0.9 rad/s, what are φ and θm?

Question

So if I substitute in omega and t=0 I have
θ = θmcos(φ). How do I solve for phi and omega center of mass???