Jerry started doing sit-ups every day. The first day he did 18 sit-ups. Every day after that he did 4 more sit-ups than he had done the previous day. Today Jerry did 26 sit-ups. Enter an equation that could be solved to find the number of days Jerry has been doing sit-ups, not counting the first day. The equation ___d + ___ = ___ can help find the number of days Jerry has been doing sit-ups.