You are looking for an equation like y=mx+a where m and a are constants.
y represents John's total earnings and x is the number of days worked in his new job.
Before actually working, John gets a signing bonus of $500, and x=0 (no days worked), so a=500 and is the y-intercept.
Every day he earns $200, so after x days he earns $200x. 200 is the slope: y=200x+500 tells you his earnings after x days' work.