You may have learned that in a/b = c/d, b and c are called means, while a and d are the extremes.
Use the fact that the product of the means equals the product of the extremes (bc=ad).
In this your case of -4/9 = 3/n, we have:
9*3 = -4n
27 = -4n
-27/4 = n
Thus in decimal, n = -6.75