x1^2-x1-1=0
x1^2=x1+1
x2^2=x2+1
x1+x2=1
x1*x2=-1
(x1)^3+7(x2)^2-5(x2)+6
=(x1+1)x1+7x2+7-5x2+6
=x1^2+x1+2x2+13
=2(x1+x2)+14=16