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A in (-oo:+oo)
(1/3)*(21-(2*A)) = 6-A // - 6-A
(1/3)*(21-(2*A))+A-6 = 0
(1/3)*(21-2*A)+A-6 = 0
1/3*(21-2*A)+A-6 = 0
1/3*(21-2*A)+A-6 = 0
1/3*A-6+7 = 0
1/3*A+1 = 0
1/3*A+1 = 0
1/3*A+1 = 0 // - 1
1/3*A = -1 // : 1/3
A = -1/1/3
A = -3
A = -3
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