If I'm correct...
The expression is not an equation. It is not solvable. I believe your expression here would be 1/3n because it is asking the relation of n feet to yards. Converting to yards from feet is to divide by 3, or multiply by the reciprocal, 1/3. So, by plugging your number of feet into the expression in place of n, you essentially divide by 3 and get your number of yards.
The equation is the actual solvable statement. You can use the expression to solve this. In the first part, you used the variable n for the number of feet. In this part, you use f instead. This new expression would be 1/3f. Now, what is that expression equal to? Well, you're solving for the number of yards. So, if the number of yards is represented by y, then the equation should be y=1/3f.