# Thread: x coordinates of horizontal tangents lines ?

1. ## x coordinates of horizontal tangents lines ?

I'm not sure if I posed the question correctly but the problem is: find the x coordinates of the points onthe graph of y=x^3-6x^2-15X+1 where the tangent lines are horizontal ?

First step:
Take the derivative which is 3x^2-12x-15

Second step:
Set the equation to zero (because that is the slope of the horizontal line)

Third Step:
What am I solving for ?

LOST !!!!

I tried to graph it in my calculator and draw the tangent lines to get a clue but no such luck

2. You've got this figured out yourself:

Originally Posted by kbgemini16
I'm not sure if I posed the question correctly but the problem is: find the x coordinates of the points onthe graph of y=x^3-6x^2-15X+1 where the tangent lines are horizontal ?

First step:
Take the derivative which is 3x^2-12x-15

Second step:
Set the equation to zero (because that is the slope of the horizontal line)

Third Step:
What am I solving for ?

LOST !!!!

I tried to graph it in my calculator and draw the tangent lines to get a clue but no such luck

$3x^2-12x-15=0$

Now solve for x.