$\\$
`x=7`
`->x+1=8`
`F=x^{15} - 8x^{14}+...-8x^2 + 8x-5`
`->F = x^{15} - (x+1)x^{14}+...-(x+1)x^2 + (x+1)x-5`
`->F =x^{15} - x^{15}-x^{14}+...-x^3 -x^2 +x^2 +x-5`
`->F =(x^{15}-x^{15})+...+(-x^2 +x^2)+x-5`
`->F =x-5`
`->F =7-5`
`->F=2`
Vậy `F=2` khi `x=7`