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c in (-oo:+oo)
((9*c)/7)*c+((6*c)/21)*c-64-21 = 0
11/7*c^2 = 85 // : 11/7
c^2 = 595/11
c^2 = 595/11 // ^ 1/2
abs(c) = (595/11)^(1/2)
c = (595/11)^(1/2) or c = -(595/11)^(1/2)
c in { (595/11)^(1/2), -(595/11)^(1/2) }
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