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x^3*((6*x^6-(21*x^4))/3) = 0
x^3*((6*x^6-21*x^4)/3) = 0
(x^3*(6*x^6-21*x^4))/3 = 0
6*x^6-21*x^4 = 0
3*x^4*(2*x^2-7) = 0
2*x^2 = 7 // : 2
x^2 = 7/2
x^2 = 7/2 // ^ 1/2
abs(x) = (7/2)^(1/2)
x = (7/2)^(1/2) or x = -(7/2)^(1/2)
3*x^4*(x-(7/2)^(1/2))*(x+(7/2)^(1/2)) = 0
(3*x^3*x^4*(x-(7/2)^(1/2))*(x+(7/2)^(1/2)))/3 = 0
( 3*x^4 )
3*x^4 = 0 // : 3
x^4 = 0
x = 0
( x+(7/2)^(1/2) )
x+(7/2)^(1/2) = 0 // - (7/2)^(1/2)
x = -(7/2)^(1/2)
( x-(7/2)^(1/2) )
x-(7/2)^(1/2) = 0 // + (7/2)^(1/2)
x = (7/2)^(1/2)
( x^3 )
1*x^3 = 0 // : 1
x^3 = 0
x = 0
x in { 0, -(7/2)^(1/2), (7/2)^(1/2), 0 }
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