4x^2-28x+y^2+6y+58=0
4x^2-28x+49+y^2+6y+9=0
(2x-7)^2+(y+3)^2=0
(2x-7)^2=0,(y+3)^2=0
x=7/2
y=-3
3x-2y
=3*7/2-2*(-3)
=21/2+6
=33/2