The answer you are looking for is "n = 0".
This can be found by adding 9 to each side, which would leave you with "4n = 0". To get "n" by itself, you'd then divide both sides by 4. "N" in this case would equal 0, because 0 divided by any number, will always be 0.
To check this answer, you could replace "n" with 0, and solve the left side. This would leave you with "-9 = -9", meaning the answer is correct.
I hope this helps!