I don't know how to solve this problem.

5. Robbie is waving a sparkler at a constant speed in a circular motion so that the tip of the sparkler is moving on a plane perpendicular to the ground (essentially just up and down). The equation h(1) = 50cos[4x(1 - =)]+ 160 models the height of the tip over time, with height expressed in centimetres and time expressed in seconds. At what times during the first 2 seconds will the tip of the sparkler be 140 cm above the ground? Round your answers to the nearest 100th of a second (9 marks) Hint: This question is worth so many marks because there is more than one time where this is true due to the periodic nature of trig functions