A crate of mass 57 kg is attached to one end of a string, and the other end of the string runs over a pulley and is held by a person as in the figure...

A crate of mass 57 kg is attached to one end of a string, and the other end of the string runs over a pulley and is held by a person as in the figure below. If the person pulls with a force of 91 N, what is the crate's acceleration? (Assume that upward is the positive direction. Indicate the direction with the sign of your answer.)