I'm having started on this problem, could somebody help me out?

According to projections by the Alzheimer's Association, the number of people suffering from Alzheimer's disease in year x is approximated by

A(x) = 4.23 * 1.023^x

where x = 0 corresponds to year 2000. When will the number of people with Alzheimer's disease be

(a) 8 million? (b) 13 million?