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x in (-oo:+oo)
3/10 = x/40 // - x/40
3/10-(x/40) = 0
3/10-1/40*x = 0 // - 3/10
-1/40*x = -3/10 // : -1/40
x = -3/10/(-1/40)
x = 12
x = 12
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