What is the stopping power of a 10kg block with an initial velocity of 10m/s which slides 10m across a horizontal surface and comes to rest after 2 seconds?

The answer is not 10 but rather 50 N,

You're trying to find a force, and we know that Force=mass*acceleration, so you need the acceleration.

Acceleration = (Final Velocity - Initial Velocity)/Time

Final Velocity = 0 m/s (it stops)
Initial Velocity = 10 m/s
Time = 2 seconds

Acceleration = (0 - 10)/2 = -5m/s^2

Now to find the force...
Mass = 10kg

Force = 10kg * -5m/s^2 = -50N