6. Let f(x, y) = ax? + bxy + cy? and g(x, y ) = f( x, y) = ax2 - bxy + cy". These forms represent precisely the same num- bers, but they are not necessarily equivalent (because the determi nant of the transformation has determinant -1). Show that x + xy + 2y' is equivalent to x2 - xy + 2y, but that 3x2 + xy + 4y and 3x2 - xy + 4y' are not equivalent