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c in (-oo:+oo)
(2.1/3)*c = 2.1/10 // - 2.1/10
(2.1/3)*c-(2.1/10) = 0
(2.1/3)*c-0.21 = 0
0.7*c-0.21 = 0 // + 0.21
0.7*c = 0.21 // : 0.7
c = 0.21/0.7
c = 0.3
c = 0.3
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