Well... I'm not entirely sure how they want you to write the equation, however:
We know that $4.50 is for one song, since we don't know how many songs we can have 'x' represent the number of songs. Finally it ends up being $50 dollars, more than likely, so... from what I would draw it would be:
$4.50x = $50 ~ Divide $4.50 by both sides.
x = 11.1 (Estimated) ~ Roughly indicating that only 11 songs can be bought.
I hope that helps in someway, have a great rest of your day! ^ ^
{-Ghostgate-}
P.S. Considering that I'm not sure how they would want you to write it, this could be wrong. However it still gives you the proper answer.