Magali exerts a force of 4.5 N to slide her Blackberry across the table (so she can hide it from Hina!). If she does 2.7 J of work in the process, how far has the Blackberry moved?

Work is equal to the magnitude of the force multiplied by the displacement x. Assuming Magali is pushing the phone, so that the force and displacement point in the same direction, then the work done is positive, and

2.7 J = (4.5 N) x   ==>   x = (2.7 J) / (4.5 N) = 0.60 m

Even if she was pulling the phone instead, the displacement would be the same, but the work done would be negative and the force would point in the opposite direction, thus also having a negative sign:

-2.7 J = (-4.5 N) x   ==>  x = 0.60 m