Skill: Calculate the perimeter of composite figures

The perimeter is the distance around the entire outside boundary of a figure. Add each exterior side once. Do not include edges inside the figure where two rectangles share a side.

Consider this L-shaped figure:

          6 cm
       ┌──────┐
       │      │
       │      │
       │      └──────┐
       │             │ 3 cm
       └─────────────┘
             10 cm

The total height on the left is $8$ cm.

Step 1: Find the missing vertical side.

The whole height is $8$ cm. The lower vertical section is $3$ cm, so the upper vertical section is

$$8-3=5\text{ cm}.$$

Step 2: Find the missing horizontal side.

The whole width along the bottom is $10$ cm. The top horizontal section is $6$ cm, so the missing middle horizontal section is

$$10-6=4\text{ cm}.$$

Step 3: Add every outside side.

Trace around the boundary and add:

$$6+5+4+3+10+8.$$

Therefore,

$$P=36\text{ cm}.$$

The perimeter of the composite figure is $\boxed{36\text{ cm}}$.