We can use the next formula
Where A is the amount, P is the principal, r is the rate n is the periods and t is the time
in our case
A=6000
n=4
r=0.08
t=15 years
We substitute in the formula
then we isolate the Principal
The principal is $1828.70
ANSWER
You need to deposit $1828.70 in an account now in order to have $6000