Answer
$y =-2 \cos (\dfrac{\pi}{2}) x +4$
Work Step by Step
General solution for Trigonometric functions $sine$ and $cosine$ that model a sinusoid is as follows: $y=A \sin B (x-h) +k $ and $y=A \cos B (x-h)+k$
Here, the amplitude is $A=2$ and we can see from the graph that it has flipped nature which means that we have $-2 \cos $ function.
Since the time period for a trigonometric function is $4$ and here $B=\dfrac{\pi}{2}$, the midline of the graph has been shifted by $4$ vertically.
Thus, we have $y =-2 \cos (\dfrac{\pi}{2}) x +4$
