$h$ is an exponential function of the form $h(n)=a\cdot r^n$ $(r>0)$. The table below lists a few values of $h$. **What is the common ratio $r$ of the function $h\ $**? $r=$ [[☃ math-keypad 1]] $n$ | $h(n)$ - | - | - $2$| $144$ $4$ | $324$ $6$ | $729$