Factorise x²+5x+6.

You need to find two numbers that multiply to make 6 and add to make 5. Usually it's easier to find a correct multiplication and check that it satisfies the addition.
Here the options are 6&1 or 2&3, so it's pretty clear that we want 2&3, so then you form the answer as so:

(x+A)(x+B) = (x+2)(x+3)