Let's start with something really simple...  If you keep this guy in your head and THINK, you'll always know how to do these.

Look at this guy:

Think about it...  What could x be?

So,

or

(We use an "or" since x can't be -3 and 3 at the same time.)

Get this guy stuck in your brain:

Here's another one:

What could x be?

So,

or

 

TRY IT:

What could x be?