Momentum must be conserved in this situation. The initial momentum is zero, so after the ball is thrown, the momentum of the ball and the person must still add up to be zero.
Lets declare direction of the baseball to be positive.
The momentum of the person must be equal to 
so that the total momentum (
) remains zero.
The velocity is negative because it points exactly in the opposite direction
they threw the ball. (This problem also assumes their skates were pointing
in the same direction they threw the ball... but thats just a silly detail)