What kind of Beta Decay is this? There are usually two kinds, where there is a positive emission or a negative emission. That might be where you made your error.
Homework Statement
A 5.00-kg block is set into motion up an inclined plane with an initial speed of 8.00 m/s. The block comes to rest after traveling 3.00m along the plane, which is inclined at an angle of 30° to the horizontal. For this motion, find the change in the block's kinetic energy...