You serve a volleyball with a mass of 2.1 kg. the ball leaves your hand with a speed of 30m/s. the ball had kinetic energy. how do you calculate that?

Your best bet is to use the formula that describes the kinetic energy
of any moving object:

          Kinetic Energy = (1/2) x (mass) x (speed-SQUARED)

In this question,
                               mass = 2.1 kg
                               speed = 30 m/s
                             speed²  =  900 m²/s²

I have overwhelming confidence that you can take it 
and finish it up from this point.

Distance^2= yellow line ^2 (see picture) = 30^2 + 49^2so distance = squareroot of (300^2 +49^2)i dont know if this is how to solve it, but i cnt think of another way