How can i write this in Matlab?

[Walter Roberson]

The summation can be split to and each of those Σ can be translated into a sum() call.

With a small bit of cleverness you can also replace two of the sums with var() calls.

Answer 1

[Ameer Hamza]

If x and y are 1D vectors then try something like this.

x = rand(100, 1);
y = rand(100, 1);

x_bar = mean(x);
y_bar = mean(y);

b_hat = sum(x-x_bar)*sum(y-y_bar)/sum((x-x_bar).^2);