Not sure how to do this one.

Question

what is the first term of a 10-term arithmetic series with t2=10 and S=120? Leave your answer in exact form

Answer 1

S₁₀ = (10/2)(2a₁ + 9d)

a₁ = a₂ - d

a₁ = 10 - d

2a₁ = 20 - 2d

S₁₀ = 5(20 - 2d + 9d)

120 = 5(20 + 7d)

24 = 20 + 7d

4 = 7d

4/7 = d

a₁ = 10 - 4/7