You need to find a common denominator. In this case the common denominator is 28. You have to multiply the bottom of the first fraction by 4 and the top (numerator) of the first fraction by 4. Your result will be 20/28. Then you have to multiplty the second fraction be 7 on the bottom and the top. Your result will be 7/28. You now have common Denominators, so you can subtract the numerators. 20/28-7/28 and arrive at your answer which is 13/28.