15(1.03^x)=18(1.05^x)

You can use any log so long as you use the same one on both sides. :)

$${\frac{{\mathtt{15}}}{{\mathtt{18}}}} = {\left({\frac{{\mathtt{1.05}}}{{\mathtt{1.03}}}}\right)}^{{\mathtt{x}}}$$

Reduce and then take the ln (natural log)of both sides.