Corelation and covariation

DX =⟨(X − µX)2⟩=⟨X2 − 2XµX + µ2X⟩=⟨X2⟩− ⟨2XµX⟩ +⟨µ2X⟩
=⟨X2⟩− 2 ⟨X⟩ µX + µ2X=⟨X2⟩− 2µ2X + µ2X=⟨X2⟩− ⟨X⟩2

Can you explain a little more about the problem you are facing?