The first five terms of an arithmetic sequence are shown below: 20, 17, 14, 11, 8, . . let n represent the term number and f(n) the term in the sequence.

For starters, find the common change in the terms, in this case each goes down by 3. This lets you know you're gonna have a -3n in your equation as each term decreases by 3. Your equation should be in f(n)=c+rn form, with r being change in f(n), or -3 in this case. This gives you f(n)=c-3n. Now, solve for c, add 3n to both sides to get f(n)+3n=c. Plug in your n of 1 and f(n) of 20 to get c=20+3(1)=23. Plug in your c to your f(n) formula to get f(n)=23-3n as your f(n) function.