Answer with Step-by-step explanation:
Let cost of Manuel's cds=m
Rita's cds originally cost three times more than Manuel's cds, but she received a $9 discount.
i.e. Rita's cds cost=3m-9
Rita spent $21 total.
i.e. 3m-9=21
Adding 9 on both sides, we get
3m=30
Dividing both sides by 3, we get
m=10
Hence, equation which can be used to find the amount Manuel spent is:
3m-9=21