A 2-kg book is held 1 m above the floor for 50 s What work is done in J?

The formula for calculating the amount of work done is W = F·s where F is the force against which the work was done and s is the displacement (in this case, height). In this case, instead of knowing the mass of the book we know the weight, which is the force with which Earth pulls the book (and the book pulls Earth). Since we know the force, we can just insert it into the equation directly.