Jerry started doing sit-ups every day. The first day he did 11 sit-ups. Every day after that he did 4 more sit-ups than he had done the previous day. Today Jerry did 49 sit-ups. Write an equation that could be solved to find the number of days Jerry has been doing sit-ups, not counting the first day. Let n represent the number of days. An equation that can help Jerry to find the number of days he has been doing sit-ups is given by

write an equation, don't solve it.