At the beggin there are 150 eggs, after making 1 omeltet, there are 144 eggs, after the second omelet, there are 138 eggs, etc.
By the given statement you can notice that each omelet demands 6 eggs. Then, after making each omeltet Al has 6 eggs lower. This can wrtten numerically as follow:
150, 144, 138, 132, 126, 120,...
That is equivalent to:
150-6, 150-6-6, 150-6-6-6, 150-6-6-6-6, 126-6-6-6-6-6, ... , 150 - 6n
where n is the nth omelet made by counting from the begining of the day.