Solve the given problem:
Problem: You have built a homemade skate board for your nephew out of a plank and a set of wheels. Your nephew speeds up to a final speed of 5 m/s on level ground. Your nephew throws himself backward off the skateboard, kicking the rickety skateboard forward at 7 m/s. Your nephew has a mass of 70kg and the skateboard's mass is 10kg.
A- How fast was your nephew moving when his feet left the skateboard?
Please use great detail in explaining the solution. Thank you.