∵x²+y²-4x+2y+5=0
x²-4x+4+y²+2y+1=0
(x-2)²+(y+1)²=0
x-2=0 y+1=0
∴x=2 y=-1
4x²-12xy+9y²
=(2x-3y)²
=(4+3)²
=49