if x=arctan(-3/4) and x+y=210 degrees, then cosy=?

if x=arctan(-3/4) and x+y=210 degrees, then cosy=? (estimate answer to two decimal places).

arctan(-3/4) means to find the angle x that makes tanx=-3/4.  knowing that,
 
cosy = cos[210 - arctan(-3/4)]
 
 
use the calculator to find the estimation.