Mariah's issue is that she did not give the fractions a common denominator. When adding and subtracting fractions, you must have the same denominator. The easiest way to do so is to multiply the denominators by each other to figure out what the denominator will be, and then multiply the numerator of each fraction by the original denominator of the opposite fraction. Then simply add or subtract the numerators and reduce. For example, If you are subtracting 2/7 from 10/9 you multiply 7 by 9 and get a common base of 63. Now to retain the same ratios for the fractions, you have to change the numerators. So you multiply 2 by 9 and 10 by 7, and now the problem should read 70/63 - 18/63. Now you subtract 18 from 70 and you should get a final answer of 52/63. Since this fraction cannot be reduced anymore the final answer is 52/63.