A spot of light on a computer screen moves in a horizontal line across the screen. At time t seconds, its distance, x mm, from the left-hand edge of the screen is given, for t>/= 0, by

x=t^3-12t^2+kt,

where k is a positive constant. Find the set of values of k for which x is an increasing function of t. (answer: k>/=48)

Im not sure whats the right way to go about solving this problem. Can somebody guide me along? Thank you!