How do I find the missing angle using sohcahtoa.

You have to use the inverse functions

for example; if

$$cos\theta=\frac{5}{8}$$   

then

$$\theta = cos^{-1}\left(\frac{5}{8}\right)$$

and, inverse cos is the same as arc cos 

so this can also be expressed as 

$$\theta = acos\left(\frac{5}{8}\right)$$

The site calculator uses the acos notation.