If using the method of completing the square to solve a quadratic function x to the second power + 8x - 35 equals 0 which number would have to be added to complete the square

Joe owns a fast-food restaurant and wants to know the average time it takes for customers to receive their orders. his restaurant serves about 500 customers a day. he timed 5 orders, and the order times he collected, given in minutes, are shown below. can the approximate order fulfillment time for all of the restaurant's customers be calculated from the given data? if so, calculate it. non-integer answers should be rounded to the nearest tenth. if no assumption can be made, type "0" in the box. the approximate order fulfillment time for all of the restaurant's customers is minutes.