Answer:
that description is already wrong. you must have mistyped something here. please read your own problem description again and then try again. if each side length of triangle B is the side length of triangle A, then they do not only share the same shape (= angles), but they are identical, and the scale factor is simply 1.
I think what you mean is that every side length of B correlates to the side lengths of A by the factor f.
Explanation:
what that means :
side a of A = f × side a of B
side b of A = f x side b of B
side c of A = f x side c of B
perimeter of A = f x perimeter of B
for any more details there has to be more information.