Heinz has been contributing $300 at the end of each month for the past 15 months to a sivings plan that earns 6% compounded monthly. What amount will he have 1 year from now if he continues the plan?

I'm not sure how to get the answer for this question which is:

$8649.11

Just use the standard annuity formula

amount = payment ((1+i)^n - 1)/i
= 300( (1 + .06/12)^27 - 1)/.=(.06/12)
= 300 ( 1.005^12 - 1)/.005
= 300(28.83037)
= 8649.11