Write a situation that can be represented by the expression 3a+6 where a=age in years

This equation can be used when comparing ages.

An example to illustrate this:
Assume that adding 6 to 3 times the age of Jack will give us the age of his grandfather.
When translating this into equations, assuming that the age of jack is "a" and the age of his grandfather is "b", we will find that:
b = 6 + 3a