Why is the denominator -12/2 and not -1/2

When a term with an exponent is moved from the denominator to the numerator, the sign of its exponent changes.
So, $\frac{1}{x^{1/2}}$ becomes $x^{-1/2}$.

The exponent in the denominator was $1/2$, not $-12/2$. The value $-12/2$ was not present in the original expression or in the calculation.