Hey I need some help on how to prove something:

Supposefis an isometry. Prove triangle ABC is congruent to trianglef(A)f(B)f(C).

I think I have to start by stating the coordinates for example triangle ABC has points A=(x1,y1) b=(x2,y2)... But I am not sure I think thatf(A) = (x'1,y'1). If anyone knows anything that would help me I would appreciate it.