By components you calculate change in momentum for each component;
Fx = dPx/dt = (m/t)[VfCos(60) - ViCos(30)]
Fy = dPy/dt = (m/t)[VfSin(60) - - ViSin(30)]
Once you get those numbers for Fy & Fx then the magnitude of the net force is;
F = SqRt[Fx^2 + Fy^2]