No way the answer can have the unit m. You deliberately apply the exponent 9/20 (0,45) to a value with unit m and multiply that by another quantity of unit m. This definitly must result in units of m^(1+9/20).
If you mean to divide by 12 m instead of just a dimensionless number 12, then you actually have to do it. How can SMath know that otherwise?

Sometimes, in engineering, you have equations which expect numbers instead of quantities (numbers times units). They come bundled with requirements of what units to use to generate the numbers from the original quantities and what the unit of the result is.

In such cases you can just divide the entry quantities by the required unit and multiply the resulting number by the specified unit. This way you document both the equation and the required units.

The following examples says that H.1 = X/28 * (LT/12)^0,45 if X and LT are entered in m and the result has the unit m.