a^2+b^2-a-4b+17/4=0,
a^2-a+1/4+b^2-4b+4=0
(a-1/2)^2+(b+2)^2=0
a=1/2
b=-2
a^b-ab
=(1/2)^-2-1/2*(-2)
=2^2+1/2*2
=4+1
=5