His error is in step 3. The variable x is being multiplied by -7 and therefore he should have divided both sides by -7, not 7 as he did.
Looks good! From what i see unless i missed something it all looks correct :D
20 + 18 = 38
38 is the perimeter
Answer:

Step-by-step explanation:
The multiplicative inverse of a number n is
, then
inverse of
is
= 
9.5 cause it brought a ten up to show the decimel