When asked to evaluate, you have to plug in the given value of n (in this case), into the given equation.
n/6 = ?
In order to find out ?, we need to know what n is.
Well, n = 12
So, plug in 12 for n
n/6 would now be written as 12/6 in order to be evaluated
Now, simplify.... What you get for 12/6 is the answer
