I'm in Algebra 2, 8th grade. This question is driving me nuts! My book and notes do not help at all. The sum of a series is 2125. The first term is 43 and the last term is 127. How many terms are there, and what is the common difference? I have no idea how to find the terms, because all of the equations I know have d in there.. and I don't know if I'm supposed to find the D first, or whatever.