We assume the deposits are made at the <em>beginning</em> of each quarter. The quarterly interest rate is 6%/4 = 1.5%. The number of quarterly payments is 15×4 = 60. The future value of an annuity due is ...
A = P(1+r)((1+r)^n -1)/r
where r is the quarterly interest rate, n is the number of payments, and P is the payment amount.