For the exponential function $f$, the table above shows several values of $x$ and their corresponding values of $f(x)$, where $a$ is a constant greater than 1. If $k$ is a constant and $f(k)=a^{49.5}$, what is the value of $k$?

x	f(x)
3	a^7
5	a^{12}
7	a^{17}