Basically ive answered part 1a but i dont understand why if Σ12 = Σ21 = 2 it shows that Σ is called the covariance matrix.
However in a 3x3 matrix like [1 2 3
7 8 9
6 5 4]
where all the numbers are different how would i determine the covariance in this case,would i put in the form 7(Σ12)=2(Σ21)=14 or isnt it valid in this case