Try this solution+explanation:
1. the common formula for this graph is y=A*cos(Bx), where A - the amplitude, B - the frequency of the given function.
2. The max. and min. vertical values are 4 and (-4), it means that the amplitude is 4 (A=-4*),
for the function y=cosx the period is 2π, it means for the given function the period is π/2, it is for its frequency 2π/(π/2)=B=4.
* if to see to the suggested graph, the max. of cos4x are in points π/4+πn, and the min. are in π*n/2, it means that it need to add '-' in the equation of the function.
3. answer: y=-4cos4x.
