How to use an array to get the complex calculation work?

[BNCOXUK]

Dear Sir,

I used the SLOPE and INTERCEPT function for a simple linear regression. Now I want to calculate the error variance with some simple formulas. I am a little stupid how the array, as I really cannot get the calculation work.

To make it easy to explain, I attached the file here. In this file, I provided the simple formula for this calculation. With the four separate steps, I got the result as 4.31. But due to my situation, I cannot have so many unnecessary steps in a file.

So now my question is how to use a single array formula to calculate the error variance based on the known values of:

Intercept (cell B1)
Slope ( cell B2)
Observed data (cells A5:C15)