Х - 26 7/21 + 100 5/28

To add these mixed numbers, first convert them to improper fractions:

26 7/21 = (2126 + 7)/21 = 553/21
100 5/28 = (28100 + 5)/28 = 2805/28

Now, add the two improper fractions:

553/21 + 2805/28

To add these fractions, find a common denominator:

21 * 28 = 588

Now rewrite the fractions with the common denominator:

(553 28)/(21 28) + (2805 21)/(28 21)
= 15484/588 + 58805/588

Now, add the fractions:

15484/588 + 58805/588 = 74289/588

Now, simplify the fraction:

74289 = 126*588 + 201
588

= 126 + 201/588

Therefore, the final answer is:

Х - 126 201/588