x²-8x+13=0
(x^4-6x^3-2x^2+18x+23)/(x^2-8x+15)
=[x^2(x^2-8x+13)+2x^3-15x^2+18x+23]/(x^2-8x+15)
=[2x(x^2-8x+13)+x^2-8x+23]/(x^2-8x+15)
=[x^2-8x+23]/(x^2-8x+13+2)
=(x^2-8x+13+10)/(0+2)
=(0+10)/2
=5