How do I prove that the variance of alpha in the equation:

y hat= alpha hat+beta hat*x(i)

is:

MSE(1/n+(x bar^2/sum of (x(i)-xbar)^2))

Assuming this is normal how would i got about actually proving that this is the variance?? Do I need to start with the expected value of y(i) and manipulate it?

(MSE is the estimator of sigma^2)