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x in (-oo:+oo)
18*e^-0.003*x+250/1 = 10.5 // - 10.5
18*e^-0.003*x+250/1-10.5 = 0
18*e^-0.003*x-10.5+250 = 0
18*e^-0.003*x+239.5 = 0 // - 239.5
18*e^-0.003*x = -239.5 // : 18*e^-0.003
x = -239.5/(18*e^-0.003)
x = -13.30555556*e^0.003
x = -13.30555556*e^0.003
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