Im having some trouble finding the critical points and relative max/min/saddle points for some questions

1. f(x,y) = 7x^2 + 4y^2

I found the critical point to be (0,0), but Im not sure how to calculate for min/max or saddle point

2. f(x,y) = e^1/2xy

Im having trouble finding the critical points and determining whether the points are max/min or saddle

3. f(x,y) = x^4 - 8xy + 2y^2 - 3

I think the critical points are (0,0), (2,4) and (-2,-4) but im not sure, and again im having trouble determining whether the points are a max/min or saddle point.

Any help or clarification would be much appreciated. Thanks!