a) A × B is the Cartesian product of A and B, given by the set
A × B = (a, b)
Then for A = {4, 6, 8} and {4, 5, 6}, we have the product
A × B = {(4, 4), (4, 5), (4, 6), (6, 4), (6, 5), (6, 6), (8, 4), (8, 5), (8, 6)}
b) The power set of a set X is the set containing all subsets of X. I'm not sure what you're asking to find the power set of, though. Hopefully it's not of A × B, since that would contain 2⁹ = 512 elements...
Instead I'll assume you mean a simpler set, such as A ∩ B. The intersection of A and B is
A ∩ B = {4, 6, 8} ∩ {4, 5, 6} = {4, 6}
and has only 2 elements, so its power set has 2² = 4 elements (much more manageable!) and is the set
Pow(A ∩ B) = {{}, {4}, {6}, {4, 6}}