A = P(1 + r/n)nt
Where:
P = the unknown initial investment
A = the final value = $15,000
r = the annual interest rate = 6% = 0.06
n = the number of times the investment is compounded per year = quarterly = 4
t = years = 4
$15000 = P(1 + 0.06/4)4*4
$15000 = P(1.015)16
$15000 = P*(1.269)
$15000/(1.269) = P
Solve for P