I suspect you mean at which points on the graph of does the graph have a normal with a slope equal to 3?
A normal to a curve is perpendicular to a tangent to the curve. Perpendicular lines have negative reciprocal slopes. So take the first derivative of your function and set that function equal to . Since you are dealing with a cubic polynomial, one would expect the derivative to be a quadratic with two zeros.
John
My calculator said it, I believe it, that settles it