If using the method of completing the square to solve the quadratic equation x^2 −19x−24=0, which number would have to be added to "complete the square"?

Acomputer store buys a computer system at a cost of $370.80. the selling price was first at $ 618, but then the store advertised a 20 % markdown on the system. answer parts a and b. a. find the current sale price.