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x in (-oo:+oo)
(-2/5)*x+12 = -42 // + 42
(-2/5)*x+12+42 = 0
54-2/5*x = 0 // - 54
-2/5*x = -54 // : -2/5
x = -54/(-2/5)
x = 135
x = 135
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