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.
