Answer:
Number of children bicycles = 117
Explanation:
Given that:
Number of adult bicycles = a = 42
Let,
x represent the number of children bicycle 
According to given statement;
The number of children bicycle is nine less three times the number of adult bicycles.
x = 3a - 9 
Putting a=42 
x = 3(42)-9
x = 126-9
x = 117
Hence,
Number of children bicycles = 117