You use explicit funtion to find a particular number in the list without needing to write all the list down.
For example, the first range of number has a constant difference of 5. Its explicit funtion would be: x(n)=3+(n-1)*5 (with x is the number you need to find, n is the number's position in the range)
Basically, a explicit funtion in that problem has the form:
x(n)=f+(n-1)*d
with:
x= the number needed to find
n= the number's position
f = the first number in the range
d = the distance
Good luck!