We start off with 21 as the first term. So a1 = 21
The common difference is d = -3 because we subtract 3 each time to get from one term to the other (eg: 21 to 18 is minus 3)
The nth term of an arithmetic sequence is found by using this formula
an = a1 + d(n-1)
let's plug in the values we have so far to get
<span>an = 21 + (-3)(n-1) </span>an = 21 - 3(n-1) an = 21-3n+3 an = -3n+24
Why do this? Well it's helpful so we can find any term in the formula we want. In this case, we want the 40th term. So just plug in n = 40 from here and you're done
