Sorry for all the math questions but i do not understand this at all. someone explain and write the equation for me ? ? they are the exact doesn't make sense to me:

the sum of three times a number (z) and 15 is equal to 21.

Frank is lending $1,000 to sarah for two years. frank and sarah agree that frank should earn a real return of 4 percent per year. instructions: a. the cpi (times 100) is 100 at the time that frank makes the loan. it is expected to be 113 in one year and 127.7 in two years. what nominal rate of interest should frank charge sarah?