Given an arithmetic sequence with a7 3 and a21 38 find a33

Given an arithmetic sequence with a_7 = -3 and a_21 = -38, find a_33. a_33 = (Simplify your answer.)

a7 = -3 ; a21 = -38

a7 = a1 + (n-1)d

-3 = a1 + 6d ----(1)

-38 = a1 + 20d ----(2)

-------------------------------- (subtract)

35 = -14d

d = -2

-3 = a1 + 6d --->a1 = -3 -6d = -3 -6(-2) = -3 +12 =9

a33 = a1+ (33-1)d = 9 + 32*(-2) = 9 -64 = -55

a33 = -55