Okay, here we have this:
Considering the provided information let's replace in the following formula:
[tex]\begin{gathered} A=P\mleft(1+\frac{r}{n}\mright)^{nt} \\ A=15000(1+\frac{0.06}{12})^{12\cdot15} \\ A=15000\cdot\: 1.005^{180} \\ A\approx$36,811.40$ \\ A-P=$36,811.40$-15000 \\ A-P=$21,811.40$ \end{gathered}[/tex]
Finally we obtain that after 15 years the interest earned is $21,811.40
