0\; \; \Rightarrow \; \; \; \underline{a>-14}" alt="x^2+y^2+z^2+2x-4y+6z-a=0\\\\(x^2+2x)+(y^2-4y)+(z^2+6z)=a\\\\(x+1)^2-1+(y-2)^2-4+(z+3)^2-9=a\\\\(x+1)^2+(y-2)^2+(z+3)^2=14+a\; \; \Rightarrow \\\\R^2=14+a>0\; \; \Rightarrow \; \; \; \underline{a>-14}" align="absmiddle" class="latex-formula">