a^2=-a+1
a^4=(a^2)^2=(-a+1)^2=a^2-2a+1=(-a+1)-2a+1=-3a+2
a^5=a^4*a=(-3a+2)a=-3a^2+2a=-3(-a+1)+2a=5a-3
a^5-5a
=5a-3-5a
=-3