8.3. In quadrilateral $A B C D$, points $E$, $F$, $G$ are the midpoints of sides $A B$, $B C$, $A D$ respectively. It is known that $G E \perp A B$, $G F \perp B C$, $\angle A B C=96^{\circ}$. Try to find the degree measure of $\angle A C D$.
$$
\text { 8.3. } \angle A C D=90^{\circ} \text {. }
$$

In $\triangle A B G$ and $\triangle B C G$, there is a height coinciding with a median, so they are both isosceles triangles. Thus, $D G=A G=$ $B G=C G$. Therefore, in $\triangle A C D$, the median $C G$ on side $A D$ is half of $A D$, so $\triangle A C D$ is a right triangle. Hence, $\angle A C D$ $=90^{\circ}$.