Find out how many days Jerry has been doing sit-ups. I tried and got 9 but I don't know about an equation
Jerry started doing sit-ups every day. The first day he did 15 sit-ups. Everyday after that, he did 2 more sit-ups than the previous day. Today, Jerry did 33 sit-ups. Write an equation that could be solved to find the number of days Jerry has been doing sit-ups.