Find the number of terms of the series 2+6+10+14+18...that will give a sum of 800.

looks like a = 2 and d = 4

(n/2)(4 + (n-1)(4) ) = 800 <---- using our standard sum formula
n(4 + 4n - 4) = 1600
4n^2 = 1600
n^2 = 400
n = 20

there were 20 terms.