Answer:
Number of CDs in Walter's collection = x = 36
Explanation:
x denotes number of CDs in Walter's collection.
Also, the number of CDs in Brian's collection is 3 times the number in Walter's collection.
Number of CDs in Brian's collection

Total number of CDs in both Walter's collection and Brian's collection = 144

So,
Number of CDs in Walter's collection = x = 36