I need to find the relationship of a scatter plot data of outside temp and the cost of air conditioning?

Answer 1

The simplest relationship will be
C = a + bT
where a is a "fixed cost" and b is cost per degree.
Using linear regression, or "least squares" method,
a = (∑C∑T^2 - ∑TC∑T) / (n∑T^2 - (∑T)^2)
b = (n∑TC - ∑T∑C) / (n∑T^2 - (∑T)^2)