Zachary purchased a computer for $1,200 on a payment plan. Two months after he purchased the computer, his balance was $960. Six months after he purchased the computer, his balance was $480. What is an equation that models the balance y after x months?
a. y = -240x + 1200
b. y = 240x + 1200
c. y = -120x + 960
d. y = -240x + 960