If using the method of completing the square to solve the quadratic equation x^2-14x+30=0, which number would have to be added to "complete the square"?

The random variable x is the number of occurrences of an event over an interval of ten minutes. it can be assumed that the probability of an occurrence is the same in any two-time periods of an equal length. it is known that the mean number of occurrences in ten minutes is 5.3. the appropriate probability distribution for the random variable