posted by Gary .
On what values of x does the graph of
f(x) = 2x^3 - 3x^2 + 12x + 87 have a horizontal tangent?
Take the derivative of the function because the derivative is equal to the slope of the tangent line.
a horizontal line has a slope of zero.
Set you derivative = 0 and solve for x. That should give you the correct value of x.