2 4/7 - 3/14 = 2 5/14
First, find a common denominator with your fractions. 7 and 14 are nice numbers to work with, because 7 is half of 14. To make 4/7 a fraction with 14 as its denominator, you double both the top and bottom values, giving you 8/14.
At this point, your equation is:
2 8/14 - 3/14
Now, you just subtract. Because 3/14 isn't a whole number, I wouldn't even worry about the 2 in 2 8/14, and I would just do 8/14 - 3/14 (we'll add in the 2 after). Because the denominators are the same, we only have to subtract the numerators. 8 - 3 is 5, so our answer is 5/14. Plus the 2, your answer is 2 5/14. If you need that as an improper fraction, it's 33/14