Question 258271
Are you solving for the zeros?
If that's not what you're asking please re-post.
{{{x^2+6x+6=0}}}
Use the quadratic formula,

{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
{{{x = (-6 +- sqrt( 6^2-4*1*6 ))/(2*1) }}} 
{{{x = (-6 +- sqrt( 36-24 ))/(2) }}} 
{{{x = (-6 +- sqrt( 12 ))/(2) }}} 
{{{x = (-6 +- 2*sqrt( 3 ))/(2) }}} 
{{{x = -3 +- sqrt( 3 ) }}} or approximately,
x=-1.26, -4.73
.
.
.
{{{ graph( 300, 300, -6, 2, -2, 6, x^2+6x+6) }}}  
.
.
.
f(x)=16-x^2
This one you can solve directly.
{{{16-x^2=0}}}
{{{x^2=16}}}
{{{x=0 +- 4}}}
.
.
.
{{{ graph( 300, 300, -5, 5, -5, 5, x^2-16) }}}