a²+2a+b²-4b+5=0
a²+2a+1+b²-4b+4=0
(a+1)²+(b-2)²=0
a+1=0 b-2=0
a=-1
b=2
于是
2a²+4b-3
=2*1+4*2-3
=2+8-3
=7