since you need at least fifty dollars, we know that the amount you buy is greater than or equal to fifty
amt you buy≥50
in other words:
number of CDs plus shipping cost≥50
now, let's call CDs x. if you pay 12 dollars per cd, then we can call the price for a 12x
12x+shipping≥50
shipping is $5, so
12x+5≥50
