Do you know what a radian is? If not, that's the problem. A radian is another measurement of angles that uses pi. 1(pi) = 180 degrees, (pi)/4 = 45 degrees, etc.
However, if you do know what radians is, this is how you do it:
31(pi)/3 is the same as 30(pi)/3 + (pi)/3, which is the same as 10pi + (pi)/3. By seeing it like this you can see that your angle is going to be in the same place as (pi)/3 because every 2(pi) you make a full revolution.
To find a reference angle it's similar. Draw a coordinate axis and plot your angle 7(pi)/4 by drawing a line. This angle is in the fourth quadrant. The reference angle is then measured from the x-axis. 7(pi)/4 is the equivalent of a "45 degree" angle in the fourth quadrant, so this means your reference angle will be (pi)/4.
I hope that helps.