1) x/12 - x/8 = x(1/12 - 1/8)
x(1/12 - 1/8) = x(2/24 - 3/24) = x(-1/24)
So, the answer is -x/24

2) 13x/21 + 9x/14 = -1
To add fractions, we need to find a common denominator, which is 42 in this case.
13x/21 = (13/21)x (2/2) = 26x/42
9x/14 = (9/14)x (3/3) = 27x/42
So, the equation becomes 26x/42 + 27x/42 = -1
(26x + 27x)/42 = -1
53x/42 = -1
53x = -42
x = -42/53

3) -3x/10 - 7/15 = x/6
To subtract fractions, we need to find a common denominator, Which is 30 in this case.
-3x/10 = (-3/10)x (3/3) = -9x/30
7/15 = (7/15) (2/2) = 14/30
So, the equation becomes -9x/30 - 14/30 = x/6
(-9x - 14)/30 = x/6
30x = -6(-9x - 14)
30x = 54x + 84
-24x = 84
x = -84/24
x = -7/2