You just have to reverse the equation, you know two of the factors, pi, and 7.3, you just make the equation pi x 7.3, and then you have the value of M.
pi x 7.3 = 22.9336263712054906.
22.9336263712054906/pi = 7.299999999999999987 (or 7.3, rounded up to the first decimal).
- JP