It says that the recipe for 9 servings would require 3/4 cup milk.
He wants to prepare recipe for 6 servings. Let's assume it requires X cups milk.
Now we can use concept of proportions to solve this problem.
The proportion of X cups for 6 servings would be same as 3/4 cup milk for 9 servings. Mathematically, we can write it as follows :-

Hence, we require
cup milk for 6 servings.