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(21-(3*x^12)+57)/7 = 0
(21-3*x^12+57)/7 = 0
(78-3*x^12)/7 = 0
-3*x^12 = -78 // : -3
x^12 = 26
x^12 = 26 // ^ 1/12
abs(x) = 26^(1/12)
x = 26^(1/12) or x = -26^(1/12)
x in { 26^(1/12), -26^(1/12) }
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