x^2-6x+8y+y^2+25=0
x^2-6x+9+y^2+8y+16=0
(x-3)^2+(y+4)^2=0
(x-3)^2=0,(y+4)^2=0
x=3,y=-4
2x-3y
=2*3-3*(-4)
=6+12
=18