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1/4-((1/7)*x)-((3/4)*x) = 71/84 // - 71/84
1/4-((1/7)*x)-((3/4)*x)-(71/84) = 0
(-1/7)*x+(-3/4)*x+1/4-71/84 = 0
-25/28*x-25/42 = 0 // + 25/42
-25/28*x = 25/42 // : -25/28
x = 25/42/(-25/28)
x = -2/3
x = -2/3
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