First term in arithmetic series is 3, last is 136, sum is 1,390, what are first 3 terms?

College algebra placement testing.  Want to know how to work this out.
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arithmetik sequens

sum=1390, first=3, last=136

averaej=(136+3)/2=139/2=69.5

sum=n*averaej...

n=sum/averaej 1390/69.5=20

delta=136-3=133

step size=133/19=7

term(n)=7*n -4

first fyu terms=3, 10, 17, 24
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