2n/5 ... let's say that each "n" is a pizza. If you have 2 pizzas, and divide them equally amongst 5 people, each person gets 2/5 of a pizza. Therefore, 2n/5 can be expressed as (2/5)n.
(-n/6) ... again, let's say that each "n" is a pizza. If you have 1 pizza, and divide it equally amongst 6 people, each person gets 1/6 of a pizza. However, given the negative sign, we must negate this fraction, so (-1/6)n.
Now, you have (2/5)n + (-1/6)n. Adding a negative number is like subtracting a positive one, so you can change it to:
(2/5)n - (1/6)n. Each quantity is in terms of some number of "n". Therefore, you can do 2/5 - 1/6, and have that many "n".
(2/5 - 1/6)n = (12/30 - 5/30)n = (7/30)n