To do this, change into common denominators. The common denominator is 21 so change the problem from 1/3 + 1/7 to 7/21 + 3/21. This gives you 10/21. It is already in simplest form.
2 and 1/3 is the whole. You need to subtract 3/4. To bring both of these to the same denominator, multiply each fraction by the others denominator. That means 1/3 is now 4/12 and 3/4 is now 9/12. They can now be subtracted. 2 and 4/12 - 9/12 = 1 and 7/12.