Question 27030
I think it's not a function.
{{{y^2 = x + 9}}}
{{{y = +-sqrt(x + 9)}}}
y has two values for every value of x I put in the equation.
For it to be a function, there can be a single value of y for each x.
I can rewrite it as:
{{{y^2 - 9 = x}}}
{{{(y + 3) * (y - 3) = x}}}
try y = +4 
{{{(4 + 3) * (4 - 3) = x}}}
{{{7 * 1 = x}}}
{{{ 7 = x}}}
now y = -4
{{{(-4 + 3) * (-4 - 3) = x}}}
{{{(-1) * (-7) = x}}}
{{{ 7 = x}}}
that's two values of y for one value of x.