해당크기의 배열을 typedef 했다고 치자궁~

typedef struct{
    float index[3][3];
}Matrix3X3;

Matrix3X3 Multiply3X3Matrices( Matirx3X3 a, Matrix3X3 b )
{
    Matrix3X3 temp;
    for( int i = 0; i < 3; i++){
        for( int j = 0; j < 3; j++){
            for( int k = 0; k < 3; k++){
                temp.index[i][j] += (a.index[i][j] * b.index[k][j]);
            }
        }
    }
    return temp;
}