ps - geometry

on second thought. the question can be done like this

Total perimeter = 24
overlapping is 3
now this overlapping can be done in many ways... but it doesn't matter because the question asks you the perimeter of remaining part.

consider these rectangles to be open threads in the form of circles. if i overlap 2 circles then the perimeter of remaining area will be total - overlapping.
hence the answer should be 21

To expand on your answer...no new lines are created (only subtracted) therefore, the entire perimiter of the triangle will be subtracted from the entire perimeter of the square.