Since 3pi/5 is not one of the normal points we learn on the unit circle let's reason through this. We know that 180 degrees on the unit circle is pi, or 5pi/5, which is the same as pi. So 90 degrees is half of 180 and therefore half of 5pi/5 which is 2.5pi/5.
Since 2.5pi/5 is at the 90 degree mark we know that 3pi/5 has to be past this so it will be in Quadrant II. I hope this helps.
