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(2/3)*(6*x-21) = 0
2/3*(6*x-21) = 0
( 2/3 )
2/3 = 0
x belongs to the empty set
( 6*x-21 )
6*x-21 = 0 // + 21
6*x = 21 // : 6
x = 21/6
x = 7/2
x = 7/2
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