Let's simplify the given expression step by step:

(a+c)(a-c) = a^2 - c^2b(2b-b) = b(2b - b) = b(2b - b) = b(2b - b) = 2b^2 - b^2 = b^2(a-b+c)(a-b-c) = a^2 - ab - ac - ab + b^2 + bc + ac - bc - c^2 = a^2 - 2ab + b^2 - c^2

Now we can substitute these results back into the original expression:

(a^2 - c^2) - b^2 - (a^2 - 2ab + b^2 - c^2)
= a^2 - c^2 - b^2 - a^2 + 2ab - b^2 + c^2
= 2ab - 2b^2
= 2b(a-b)

Therefore, the simplified expression is 2b(a-b).