a) To add fractions, we need to find a common denominator. The common denominator for 4 and 3 is 12.

So, we have:

1/4 + (-2/3) = (1/4) * (3/3) + (-2/3) * (4/4)
= 3/12 + (-8/12)
= (3 - 8)/12
= -5/12

Therefore, a = -5/12.

b) To add fractions, we need to find a common denominator. The common denominator for 54 and 126 is 378.

So, we have:

-24/54 + (-35/126) = (-24/54) * (7/7) + (-35/126) * (3/3)
= -168/378 + (-105/378)
= (-168 - 105)/378
= -273/378

We can simplify this fraction by dividing both the numerator and denominator by their greatest common divisor, which is 3:

-273/378 = (-273/3) / (378/3)
= -91/126

Therefore, b = -91/126.