Question 91948
{{{f(a) = (a^2-16)/( a^2-3a-28)}}}

Let's simplify this expression first

{{{f(a) = (a+4)(a-4)/(a-7)(a+4)}}}

here a+4 canceled.

Now we have {{{f(a) = (a-4)/(a-7)}}}

Domain is a collection of all allowed x values.

in the above expression we have a -7 on the bottom, you know

it can not be 0, so a -7 can not be 0, that means a can not equal to 7,

the answer is a can be any real numbers except 7.

the graph of {{{f(a) = (a^2-16)/ (a^2-3a-28)}}} can verify the answer

{{{graph(200,200, -10, 10, -10, 10, (x^2-16)/(x^2-3x-28))}}}