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(1/5)*(5*x-15) = (2/7)*(14*x-21) // - (2/7)*(14*x-21)
(1/5)*(5*x-15)-((2/7)*(14*x-21)) = 0
(1/5)*(5*x-15)+(-2/7)*(14*x-21) = 0
1/5*(5*x-15)-2/7*(14*x-21) = 0
1/5*(5*x-15)-2/7*(14*x-21) = 0
3-3*x = 0
3-3*x = 0
3-3*x = 0 // - 3
-3*x = -3 // : -3
x = -3/(-3)
x = 1
x = 1
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