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x in (-oo:+oo)
7*((1/14)*x-((8/21)*y)) = 0
7*((1/14)*x+(-8/21)*y) = 0
7*(1/14*x-8/21*y) = 0
( 1/14*x-8/21*y )
1/14*x-8/21*y = 0 // + -8/21*y
1/14*x = -(-8/21*y) // : 1/14
x = (-(-8/21*y))/1/14
x = 16/3*y
( 7 )
7 = 0
x belongs to the empty set
x = 16/3*y
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