whenever you add or subtract fractions, the denominators must be the same.
in the first problem, the denominators are the same so we just add the top numbers. so 4/7 + 1/7 = 5/7.
in the second, they again all have 9 on the bottom so we can add the top. so 2/9 + 3/9 + 1/9 = 6/9. However 6/9 can be reduced. 6 and 9 are both divisible by 3. so after we divide them we get 2/3 for our answer.
finally, the denominators are the same so we can just subtract the top. 7/8 - 2/8 = 5/8.