the US census bureau

According to data from the US census bureau, the number of retail drug prescriptions (in millions) can be approximated by:

y=83.25x + 2920.94,

where x = 0 corresponds to the year 2000, find the approx. number of prescriptions for

a) 2004

b) 2009

c) in what year will the number of prescriptions be 4,750,000

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I understand that:

A) 3235.95

B) 3670.19

But how do I determine C? Please breakdown