The function is 

Apply derivative on each side with respect to
.



\
.
The condition for horizontal tangent is 
Substitute
in the equation
.

and 
Therefore the horizontal tangent the point are
and
.
The condition for vertical tangent is
.
Substitute
in the equation
.

and
.
Therefore the vertical tangent the point are
and
.
Hence the horizontal tangent the point are
and
.
And the vertical tangent the point are
and
.