6x

__
12x

6x/(12x)

would be simplified just like an ordinary (numeric fraction).
Look for common factors in the numerator and denominator to cancel out, separately for numeric values and variables.

For the numeric part, the HCF is 6 and for variables, it is x.
So divide both numerator and denominator by 6x to get
6x/(12x)
=1/(2)
=1/2