Develop regression model while given "average" data

I'm stuck on the following question and would like some guidance please:

From a study, the average heights of Grade 5, Grade 6 and Grade 7 boys are shown below:

Grade 5

Grade 6

Grade 7

The sample size was 1500 boys.

Develop a regression model that will allow you to compare the heights of the three grade levels.

I figure that my x variable is my grade level which I will denote as Grade 5 = 1. Grade 6 = 2, Grade 7 = 3.

The y variable will be the heights in cm.

But why am I given the sample size of 1500 boys? Isn't my "n" value the count of the grades which is 3? What do I do with the 1500? If I use 1500 as my "n", don't I need 1500 sample points?

Some guidance would be appreciated.

Thank you
I'm sorry. I am also given that the "heights were standardized using z-scores" and are:

Grade 5

Grade 6

Grade 7

But I dont see how this helps. With z and the mean, I can get the std deviation. But why do I need that in a regression question? I should just need the x and y values and the value for n.....

Am I missing something?