You cannot use a negative number number of therms, so we would establish the lower bound at 0. Then we observe that at 50 therms the marginal rate, or the charge per therm changes. So, using the given information, we could write:

$\displaystyle C(x)=\begin{cases}0.73006x+15.95, & 0\le x\le50 \\[3pt] 0.4998x + 27.463, & 50<x \\ \end{cases}$

Here's a graph of the function (the domain of the top piece is shaded in red and the domain of the second is shaded in green, where I've added an upper bound of 150 therms):