6 Divide each side of the square into 8 equal parts, and take the division points as vertices (excluding the vertices of the square), the number of different triangles that can be formed is ( ).
(A) 1372
(B) 2024
(C) 3136
(D) 4495
Method 1: First, note that the three vertices of the triangle are not on the same side of the square. Choose any three sides of the square so that the three vertices lie on them, which can be done in 4 ways; then select a point on each of the chosen three sides, which can be done in $7^{3}$ ways. There are $4 \times 7^{3}=1372$ such triangles. Additionally, if the triangle has two vertices on one side of the square and the third vertex on another side, first choose one side to have the two vertices of the triangle, which can be done in 4 ways, then choose any two points on this side, which can be done in 21 ways, and then choose any one point from the remaining 21 points as the third vertex. There are $4 \times 21 \times 21=1764$ such triangles. In summary, the number of different triangles is $1372+1764=3136$. Therefore, the answer is C.
Method 2: $\mathrm{C}_{28}^{3}-4 \mathrm{C}_{7}^{3}=3136$. Therefore, the answer is C.