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