Step 1: Recall what it means to lie on the x-axis.
A point lies on the x-axis exactly when its y-coordinate is 0, so we only really need to track the y-coordinates of the two given points.
Step 2: Look at just the y-coordinates first.
The given points are $(5,-4)$ and $(6,4)$; their y-values, $-4$ and $4$, are exact opposites, so averaging them gives $\frac{-4+4}{2}=0$ before we even touch the x-coordinates.
Step 3: Confirm with the full midpoint.
Averaging the x-coordinates too gives $\frac{5+6}{2}=5.5$, so the midpoint is $(5.5,0)$, which has y-coordinate 0 and therefore lies on the x-axis.
\[ \boxed{\text{x-axis}} \]