How do you write "Twice the the quantity a number and 3."?
a number and 3:
if the number is denoted
to have twice this whole quantity (instead of, for instance, only n being doubled:
then to double it, put a 2 before the whole expression (n+3)
the final expression:
2(n + 3) is probably what you are looking for.
Twice the quantity usually means, "get ready for parentheses".
Inside the parentheses, expect a sum when the statement just says "and". So I would write 2(n + 3).