Question 797112
{{{ x^2 - 9 = 0 }}}
The quadratic formula applies to a parabola
which has the form:
{{{ a*x^2 + b*x + c = 0 }}}
The formula is:
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
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You can make your equation into this form:
{{{ 1*x^2 + 0*x - 9 = 0 }}}
{{{ a = 1 }}}
{{{ b = 0 }}}
{{{ c = -9 }}}
Now plug these values into the formula
{{{x = (-0 +- sqrt( 0^2-4*1*(-9) ))/(2*1) }}} 
{{{ x = ( 0 +  sqrt( 36 )) / 2 }}}
{{{ x = 6/2 }}}
{{{ x = 3 }}}
and, taking the negative square root,
{{{  x = ( 0 - sqrt(36 ) ) / 2 }}}
{{{ x = -3 }}}
The solutions are {{{ x = 3 }}}, {{{ x = -3 }}}
check:
{{{ x^2 - 9 = 0 }}}
{{{ 3^2 - 9 = 0 }}}
{{{ 0 = 0 }}}
and
{{{ (-3)^2 - 9 = 0 }}}
{{{ 0 = 0 }}}
OK