You start by looking for the words "a number".

That's N  (or any letter you choose, capital or small)

Then you look at "twice a number"

That means to multiply the number N by 2

That's 2N

Then you look at "3 more than twice a number""

To make something 3 more you add on 3 with a plus
sign and a 3.

Answer:

2N + 3

Edwin