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(-2/5)*x+(3/7)*x+3 = 841/280 // - 841/280
(-2/5)*x+(3/7)*x-(841/280)+3 = 0
(-2/5)*x+(3/7)*x-841/280+3 = 0
1/35*x-1/280 = 0 // + 1/280
1/35*x = 1/280 // : 1/35
x = 1/280/1/35
x = 1/8
x = 1/8
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