To find d, let the a₄= a₁.
an= a₁ + (n-1) d
54= 79 + (6-1) d *6 since we started to count the terms in a₄*
54 = 79 + (5)d
54 - 79 = 5d * Transpose; combine like terms*
-25= 5d * Divide both sides by 5*
5
-5 = d
Find a₁:
an= a₁ + (n-1) d
54 = a₁ + (9-1) -5
54 = a₁ + (8) -5
54 = a₁ + (-40)
54 + 40 = a₁
94 = a₁
find a₂₀:
an= a₁ + (n-1) d
a₂₀ = 94 + (20 - 1) -5
= 94 + (19) -5
= 94 + (-95)
= -1
