So he gets paid $200 as a base rate, and $12 dollars more per album sold, so lets name the albums sold x for that we can just plug in whatever number for however many albums have been sold. So now you have 12x+200.
He made $644, so make 12x+200=644, then solve.
I got x= 37.
Plug in to check.
Should be right if you need to solve after.