Answer:
16
Step-by-step explanation:
If you know that length times 2+width times 2 is the perimeter then you multiply the length, 18, by 2 you get 36 then you subtract that from 68 which is 32 then you divide that by 2 and you get 16. You check it by using the formula L2+W2=68 which would be 36+32= 68.
hope this helped
