if sinx = y then x = arcsiny
if cosx = y then x = arccosy
if tanx = y then x = arctany
and so on
then you usually have to add k*pi, or 2k*pi depending on what interval you want to solve for and the particular problem. these functions are periodic, so they repeat the same values over and over, that's what the multiples of pi take care of
do you understand how they ended up with tan@ = -2/3?