ok, by pluggin in a few numbers, here's what i could come up with..let me know if it is wrong

n/2(2f+(n-1)d)

where n: # of terms

f: first term in sequence

d: difference between consecutive terms

arriving at number of terms in sequence has a formula that i knew of..

n = (l - f)/d

where n: num terms

l: last

f: first

d: difference