Rearranging trigonometry formula How do I rearrange this trig formula to make y the subject? I am having trouble to take the $y$ out of the cosine.
$x = \cos(a+y)\times50$
Any help is appreciated.

$$x = \cos(a+y)\times50$$
Divide both sides by $50$:
$$\frac{x}{50} =\frac{ \cos(a+y)\times50}{50}$$
$$\frac{x}{50}=\cos(a+y)$$
Now take the inverse function of cosinus:
$$a+y=cos^{-1}\left(\frac{x}{50}\right)$$
Subtract by $a$:
$$y+a-a=cos^{-1}\left(\frac{x}{50}\right)-a$$
This means that 
$$y=cos^{-1}\left(\frac{x}{50}\right)-a\:.$$
