Students often struggle computing the correct values of Perimeter and Area L-Shaped Polygons since all the sides are NOT given. The student must translate distances and compute the missing side.
This process is easy to do with a little practice. The teacher can divide the L-Shape into two rectangles and transfer the dimensions, and students quickly realize that it is either an addition or subtraction problem to ensure the dimensions are compatible.
These three (3) practice sheets will solve those initial issues in a couple class days of focused practice.
Best to you and your students.