Questions on multiple linear regression models

Questions on multiple linear regression models: Suppose we fit the model y = X1β1+ ε when the true model is actually given by y = X1β2 + X2β2 + ε. For both models, assume E(ε) = 0 and Var(ε) = σ2I. where I is the identity matrix. Find the expected value and variance of the ordinary least-squares estimate, .  (Note here X,I are matrices, and y,β,ε are vectors since those are multiple linear models)