Factoring is just the following process...

We do not know "h", so we do not know "h+8" etc,

hence we multiply out "term-by-term".

Your equation is

which is also saying

Both are equal, so if you subtract them, the result is 0.

You can subtract either side, so you can have

or

Hence, if you a "minus" on the squared term, you can always change all the signs

when you have an equality.

so you can factor using

Hence, you need the factors of -40 that combine to give -3.

If you have "negative h squared", your factors look like this...

and you must arrive at

so you need the factors of 40,

such that multiplying out the brackets leaves you with +3h

so