A) To subtract fractions, the denominators must be equal first. In this case, 5, 7, and 10 are not equal.

To find a common denominator, we can find the least common multiple (LCM) of 5, 7, and 10 which is 70.

Now we can rewrite the fractions with the common denominator:
2/5 = 28/70
6/7 = 60/70
1/10 = 7/70

Now we can subtract the fractions:
28/70 - 60/70 - 7/70
= (28 - 60 - 7) / 70
= (-39) / 70

So, the result of A) 2/5 - 6/7 - 1/10 is -39/70.

B) To divide fractions, you must multiply by the reciprocal of the second fraction.

So, -1/2 divided by -3/4 is the same as (-1/2) * (-4/3) = 4/6 = 2/3.

So, the result of B) -1/2 : (- 3/4) is 2/3.