Question

Determine the point at which the graph of the function below has a horizontal tangent line. (If the function has no horizontal tangent line, enter NONE.)
y =x3 + 3x

Answer 1

But the answer is possibly 9

Answer 2

f ( x ) = x³ + 3 x
Slope of a horizontal line is 0.
f ` ( x ) = 3 x² + 3
3 x² + 3  = 0
3 x² = - 3      / : 3
x² = - 1 ,  which give complex roots.
Answer:  NONE