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But why doesn't it work in this problem?

I did (using your method)problem said:A constant force 85N accelerates a 12kg box from a speed of 2.0m/s to a speed of 8.0m/s as it travels 15m along a horizontal surface. What is the coefficient of friction between the two surfaces?

ΔKE + ΔPE + ΔTE = 0

ΔKE + ΔPE - W = 0

½m(v² - v₀²) + mg(h - h₀) - Ffr(x - x₀) = F • (x - x₀)

½(12)(8² - 2²) + 0 - µ(12)(9.8)(15 - 0) = 85N • (15 - 0)

µ = -0.51

So why doesn't it work on this problem?