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x in (-oo:+oo)
56 = 7*x+5*x+8 // - 7*x+5*x+8
56-(7*x)-(5*x)-8 = 0
56-7*x-5*x-8 = 0
48-12*x = 0 // - 48
-12*x = -48 // : -12
x = -48/(-12)
x = 4
x = 4
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