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x in (-oo:+oo)
m = (3*(u+1))/7-((2*(u-2))/21)-((2-u)/3) // - (3*(u+1))/7-((2*(u-2))/21)-((2-u)/3)
(2*(u-2))/21-((3*(u+1))/7)+(2-u)/3+m = 0
(-3/7)*(u+1)+(2*(u-2))/21+(2-u)/3+m = 0
(2*(u-2))/21-3/7*(u+1)+(2-u)/3+m = 0
(-3/7*3*21*(u+1))/(3*21)+(2*3*(u-2))/(3*21)+(21*(2-u))/(3*21)+(3*21*m)/(3*21) = 0
2*3*(u-2)-3/7*3*21*(u+1)+21*(2-u)+3*21*m = 0
63*m-21*u-21*u-39+42 = 0
63*m-42*u+3 = 0
(63*m-42*u+3)/(3*21) = 0
(63*m-42*u+3)/(3*21) = 0 // * 3*21
63*m-42*u+3 = 0
x belongs to the empty set
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