To simplify the expression, first distribute the terms in the numerator:

(b^2 - 4*b + b^2) = 2b^2 - 4b

Now, the expression becomes:

(2b^2 - 4b)/(b^2 - 4)

Next, factor out a 2b from the numerator:

2b(b - 2)

And factor out a (b - 2) from the denominator:

(b - 2)(b + 2)

Therefore, the simplified expression is:

2b/(b + 2)