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||\vec{v}_{f}||=\frac{1}{m_{1}+m_{2}}\sqrt{(m_{1}v_{1}-m_{2}v_{2}\cos(30))^{2}+m_{2}^{2}v_{2}^{2}\sin^{2}(30)}

here is the problem Dan is gliding on his skateboard at 3.00 m/s. He suddenly jumps backward off the skateboard, kicking the skateboard forward at 8.00 m/s (as measured by an observer on the ground). Dan's mass is 40.0 kg and the skateboard's mass is 5.00 kg. How fast is Dan going as his feet hit the ground?

I am just not sure which equation to use.