Recognise that the intial potential energy and kinetic energy at 10 m/s position should be identical due to the frictionless slope.
mgh = 0.5mv2
Step 1: Recognise 4cos(2x )+ 2sin(2x) must be put into the form given in the question form i.e. Rcos(2x - a) as it contains only one trigonometric function
Step 2: Calculate values of R and a