The function is .

Apply derivative on both sides.

.

The slope of the horizontal tangent is 0.

Evaluate .

and 

and 

and

.

Find the points at which the function has horizontal tangent line.

Substitute in .

Point is .

Substitute in .

Point is .

Substitute in .

Point is .

Points at which the function has horizontal tangent lines are , and .

Points are , , .