Let n be the number of days he has been doing sit ups, not counting the first day. Since he does 2 more sit-ups for each day, then the number of additional sit-ups is 2n.
Since he started doing 15 sit-ups on the first day, the total number of sit-ups
after x days would be 2n+15.
Since he did a total of 33 sit ups today, then 2n+15=33
