Answer
$ y=\displaystyle \frac{\log x}{\log 15}$
Work Step by Step
Assuming that the graphing utility does not have defined logarithms with bases other than 10 or e, we apply the Change of base property: $\displaystyle \quad \log_{b}M=\frac{\log_{a}M}{\log_{a}b}$
Change to base 10 (common logarithm)
$ y=\displaystyle \log_{15}x=\frac{\log x}{\log 15}$
See graph.
