Dan is gliding on his skateboard at 4.5 m/s. He suddenly jumps backward off the skateboard, kicking the skateboard forward at 8.0 m/s. How fast is

Dan is gliding on his skateboard at 4.5 m/s. He suddenly jumps backward off the skateboard, kicking the skateboard forward at 8.0 m/s. How fast is Dan going as his feet hit the ground? Dan's mass is 45 kg and the skateboard's mass is 5.0 kg.