I'm new to arithmetic progression and i'm looking for a little help. i've been given the following example:

q. determine an expression for the n'th term for
15,19,23......
use your answer to determine the 80th term

a. a=15 d=4
n'th term = a+(n-1)d
= 15+(n-1)4
= 15+4n-4
n'th term = 11+4n
80th term = 11+4x80
= 331



i can't understand how 15+(n-1)4 becomes 15+4n-4