(x+3)2-9

One way to do it, until you learn a shorter way,
is to first substitute a single letter, like A,
for the part in parentheses, (x+3), and if you 
need to, you can write 9 as 32.

Then it becomes

A2-32

which factors as

(A-3)(A+3)

Now change the parentheses ( ) to brackets [ ]

[A-3][A+3]

Now put (x+3) in place of the A's

[(x+3)-3][(x+3)+3]

Now remove the ( ) inside

[x+3-3][x+3+3]

Combine like terms:

[x][x+6]

Erase the brackets around the x and change
the brackets around the x+6 to parentheses:

x(x+6)

After you've done enough of these, you can 
shorten the process.

Edwin