Write $an$ equation $of$ the cosine function with amplitude $2$ and period $6.$

$y=-2cos(\frac{1}{6}x)$

$y=-\frac{1}{2}cos(\frac{1}{3}x)$

$y=2cos(\frac{1}{3}x)$

$y=\frac{1}{2}cos(\frac{1}{6}x)$