You can make the equation y=4x+120
This is from the formula y=mx+b
Y is the total amount
M is the rate
X is the time
B is the initial value
The information you provided says
Y is the total amount of songs
M is 4 because 4 songs are being
added each week
X is the number of weeks
B is 120 because Iris already had that
many songs in the beginning