Integration/differentiation

Only just started inegration and differentiation and to top it off, the queston

has totally thrown me off track, any help through the steps would be greatly appreciated.

The number of sales, S, in 000's of a phone is given by the formula:

S=Ae^kt

where t is time in weeks after the launch. In the week leading to the launch

3000 phones are ordered (ie when t = 0 then S = 3000)

In the fifth week (ie at t = 5) 5000 are sold.

(i) find A and k

(ii)predict, according to this model, what the phone sales will be after 7

weeks.

(iii) How long will it take to reach 20000 per week?

thankks