Answer: y = 3.5cos(x)
Explanation:
We identify cos(x) and sin(x) by looking at what the shape looks like from 0 to infinity. If the wave starts at its peak at x = 0, then it's a cos(x) graph, and if it starts at the middle of its height, it's a sin(x) graph.
In this case, the wave starts at its peak at x = 0, which means it's a cos(x) graph, and the way we can find "a" is by looking at the y-intercept, as at cos(0), cos(0) = 1. From the graph, we can see that the y-intercept is 3.5, which means our final answer is y = 3.5cos(x).