Help please ASAP this is confusing first correct answer gets brainleist

8cm
First we need to look at the side opposite to the missing side
which it is 12cm
now we can see that on the missing side 4cm is right below it
so we do
12cm-4cm
this gives you
8cm
a) 12-4=8
b) 5+4+4+12+9=34
The perimeter is 34cm and the missing length is 8cm.
Or it could be 38 since this unknown length was determined.