Question 107317
 a "function" must be {{{single-valued }}}  which means "one and only one"; to each value of {{{x }}}there is a {{{unique}}} value of {{{y}}}.

The values that {{{x}}} may assume are called the {{{domain}}} of the function, and those are the values for which the function is {{{defined}}}.
In the function {{{ y = ax + b}}}, the {{{domain}}} may include {{{all }}}{{{real}}}{{{ numbers }}};  consequently, {{{x}}} could be {{{any}}}{{{ real}}}{{{ number}}}. 

But, there is {{{one}}} case in which the domain {{{must}}} be restricted, and that is the case when a denominator {{{may}}}{{{ not}}}{{{ be}}}{{{ 0}}}.  

So, in this function {{{y = 2 /( x+6)}}} a denominator {{{x+6}}} may not be equal to {{{0}}} because to divide by {{{0}}} doesn’t make a sense.

Let set it equal to {{{0}}} in order to find what {{{x}}} may not be in a domain:

{{{x+6 = 0}}}
{{{x = 0 – 6}}}
{{{x = -6}}}

Solution:

In a domain could be all other real numbers except {{{-6}}}.

Domain: ({{{- infinity}}}, {{{-6}}}] and [{{{-6}}},{{{+infinity}}})