You have that each song costs 0.50$ and each album costs 3$. If Jada only has 21$, and s is the number of songs and a the number of albums, then, 0.50s is the cost of s songs and 3a is the cost of a albums.
The sum of 0.50s and 3a must be equal to 21, due to this is the amount of money Jada has.
Hence, the equation is:
0.50 + 3a = 21
