Since the annual rate of 4.5% is compounded monthly, the monthly interest is (4.5/12)% or .375%. If we started with $2800, after 1 month we would have:

1 month= 2800(1+.00375)=2810.5

We can find successive months using:

Amount at end of month= 2800(1+.00375)^n

where n is the number of months. To determine how long it will take to grow 2,800 to 8400, we will use:

8400=2800(1+.00375)^n

Solving for n, we get:

n=293.512 months

Since the interest is only given at the end of every month, we will need to round this up to 294 months. This is how long it will take.