so the first thing you would have to is to find a common denominator so a common denominator between 11 and 15 is 165. 3/11 * 15 =45/165 next you would do 7/15 * 11=77/165 no that both denominators are the same we add the numerator but the denominator stays the same so 45/165 + 77/165 = 122/165
and since it can't be reduced any more 122/165 is your answer.
hope this helps :)