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(1/3)*x+3/7 = 10/21 // - 10/21
(1/3)*x+3/7-(10/21) = 0
(1/3)*x+3/7-10/21 = 0
1/3*x-1/21 = 0 // + 1/21
1/3*x = 1/21 // : 1/3
x = 1/21/1/3
x = 1/7
x = 1/7
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