$\"\"$

The function is $\"\"$ .

Apply derivative on both sides with respect to $\"\"$ .

$\"\"$

$\"\"$ .

$\"\"$

The slope of the horizontal tangent is 0.

Evaluate $\"\"$ .

$\"\"$

Substitute $\"\"$ in the function $\"\"$ .

$\"\"$

Point is $\"\"$ .

The function has a horizontal tangent line at $\"\"$ .

$\"\"$

The function has a horizontal tangent line at $\"\"$ .