How would I find the formula for the nth term of a sequence such as 3, 7, 11, 15, 19?

This is an example of a linear sequence which will always have the formula: u_n=an+b.Where 'a' is the common difference between each term and 'b' is the 'zeroth' term.By subtracting the previous term from each individual term it can be found that the different is 4, (7-3=4, 11-7=4 etc.)and to find b, if 3 is the first term the zeroth term can be found by subtracting the common difference from the first term:3-4=-1.Hence the formula for the nth term is, u_n=4n-1