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a = - 5/21 + ( - 3/ - 18)
a = - 30/126 + ( - 21/126)
a = - 30/126 - 21/126 
a = - 51/126
a = - 17/42

b =  - 7 /42 - 8/30
b = - 35/210 - 56/210
b = - 91/210 =  - 13/30

c = - 3/10  - 1/15 
c = - 9/30 - 2/30
c = - 11/30

-5/21+-3/-18
= -5x6/21x6+3x7/18x7
= -30/126+21/126
= -9/126
= -1/14

-7/42 + -8/30
= -1/6 - 8/30
= -5/30-8/30
= -13/30

-3/10+1/-15
= -3x3/3x10 + 2/-15x2
= -9/30-2/30
= -11/30