If using the method of completing the square to solve the quadratic equation x^2+14x-1=0, which number would have to be added to "complete the square"?

At a pizza shop 70% of the customers order a pizza 25% of the customers order a salad and 15% of the customers order both a pizza and a salad if s customer is chosen at random what is the probability that he or she orders either a pizza or a salad