add formulas to complete the table of hours used. in cell b17, create a nested formula with the IF and SUM functions that check if the total number of hours worked in week 1 (cells b9:f9) is equal to 0. If it is, the cell should display nothing (indicated with two quote marks: '' ''). Otherwise, the cell should display the total number of hours worked in week 1. Copy the formula from cell b17 to fill the range b18:b20.