Hey guys, its my first post here and I was wondering if you could help.

I need to show the point on the graph of:

x^2-3xy+y^2=1

has a horizontal tangent line.

I am assume that since I know when its horizontal = 0 for slope and y'=0

ps (when I am writing y' it = yprime)

So step one is find the derivative.

The teacher told us the derivative is 2x-3y-3xy'-2yy'=0

is this right ? can anyone explain it?