Explanation:
to find the missing side length we need to use the invisible triangle that is kind of cut off in the top left corner.
it is a right-angled triangle, and the missing side is the Hypotenuse (baseline). therefore, we use Pythagoras.
now, can we get the invisible side lengths of this triangle ?
yes, because the complete figure including the invisible triangle is s perfect rectangle with 2 equal lengths and 2 equal widths.
so, the complete rectangle side on the left would also be 5. but we see only 2. so the invisible part is 5-2=3.
and the complete rectangle side in the top would also be 8. but we see only 4. so the invisible part is 8-4=4.
so, with Pythagoras,
unknown side² = 3² + 4² = 9 + 16 = 25.
unknown side = 5
and now the perimeter is
P = 2 + 8 + 5 + 4 + 5 = 24