Prove that $\displaystyle log_ax=\frac{lnx}{lna}$

The reason I'm asking this is because I was sleeping through logs. I'm now in calculus and in the proof of $\displaystyle \frac{d}{dx}(log_ax)=\frac{1}{(lna)x}$, they rewrite $\displaystyle log_ax$ as $\displaystyle \frac{lnx}{lna}$. I need to understand this so that I may sleep tonight. Thanx.