左边=x²-6x+y²+8y+9+16
=(x²-6x+9)+(y²+8y+16)
=(x-3)²+(y+4)²=0
∵（x-3）²≥0 (y+4)²≥0
∴ x-3=0 y+4=0
∴ x=3
y=-4