$h$ is an exponential function of the form $h(x)=a\cdot r^x$ $(r>0)$. The table below lists a few values of $h$. **What is the initial value $a$ of the function $h\ $**? $a=$ [[☃ numeric-input 1]] $x$ | $h(x)$ - | - | - $1$| $3$ $2$ | $15$ $3$ | $75$