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2/3-(((7/3)*x^8)/21) = 0
(-1/9)*x^8+2/3 = 0
-1/9*x^8 = -2/3 // : -1/9
x^8 = 6
x^8 = 6 // ^ 1/8
abs(x) = 6^(1/8)
x = 6^(1/8) or x = -6^(1/8)
x in { 6^(1/8), -6^(1/8) }
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