Step-by-step explanation: first you write the expression which is $1.19X + 4.99 because $1.19 is the cost of per song download and X stands for the numer of songs .
then after you khow the expression you substitute ( which meanse to put something instead of a variable like X) 9 because 9 is the total number of song
that how you get $15.7 if you substutute 9 with X.
