1. The problem is to solve the equation $2 - 6 = \frac{\boxed{\quad}}{3}$, where the numerator of the right side fraction is unknown. 2. Simplify the left side: $2 - 6 = -4$. 3. So the equation becomes $-4 = \frac{\boxed{\quad}}{3}$. 4. Multiply both sides by 3 to solve for the numerator: $-4 \times 3 = \boxed{\quad}$. 5. Calculate: $-4 \times 3 = -12$. 6. Therefore, the missing numerator is $-12$. **Final answer:** $\boxed{-12}$.