multivariable calculus critical points

Apr 2017
165
6
New York
Hello everyone,
in this question I tried to the critical points.

what should be my next step?

I could not come up any conclusion :(
 

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Sep 2016
793
540
USA
Do you know what the definition of a critical point is?
 
Apr 2017
165
6
New York
I solved that question now. You can ignore it. Thanks.