Stationary points occur when dy/dx = 0 i.e. 6cos 2x - 2sin 2x = 0
Now, so long as cos 2x doesn't equal 0 you can divide by it to get 6 = 2tan 2x or tan 2x = 3 as required.
To finish off, just confirm that IF cos 2x does equal 0 then you don't get a stationary point, i.e. you haven't inadvertently eliminated any stationary points by dividing by cos 2x.