last arithmetic sequence one

t₄  = -5

t₉ = 10

We have this

t₉ =  t₄  + d ( 9- 4)

10  = -5  + d (5)      add -5 to both sides

15  =  d (5)            divide both sides by 5

3  =  d

To find t₁   we have

t₄  = t₁  + d ( 4 - 1)

-5  = t₁ + 3 ( 3)

-5 = t₁ +  9          subtract  9 from both sides

-14  =  t₁