Question

How do you find `a_21` given 121, 118, 115, ...?

Answer 1

If `a(1)=121`, `a(2)=118`, and `a(3)=115`, we can create an equation for `a(n)`, then plug in `n=21` to get `a(21)`.

`a(n)=-3n+b`
`121=-3(1)+b`
`b=124`

`a(n)=-3n+124`

`a(21)=-3(21)+124`
`=61`