Just over 7 hours? If you want a quicker answer, try a dictionary. There are regular users who are still asleep!
:hi: Welcome to the forum.
I take it you're talking about the verbs - 'notice' can be a noun too. Generally, there's a lot of overlap; sometimes you can use either; in these cases, formality makes the choice. - 'observe' is more suited to formal contexts.
But, to my ear, 'observe' can have more of a sense of intentional observation; in contrast, you can 'notice' something without meaning to.
'I couldn't help noticing that you were on your own. Do you mind if I join you?' (No intention; but -)
I observed a man in a blue coat behaving suspiciously. (The speaker had a good look.)
However, these two are equally acceptable; the only difference is in the degree of formality -
He noticed a letter on the table. (Less formal) He observed a notice on the table. (More formal)
But you really need to use a dictionary to find out more. For example, 'observe' can mean 'say'.