(x-3)^2非负,(3x+y+5)的平方根非负
(x-3)^2与(3x+y+5)的平方根互为相反数→
x-3=0,(3x+y+5)=0→
x=3,(3*3+y+5)=0,→y=-14
∴5x-y+2=5*3+14+2=31