You can't do anything with this problem if you don't remember what "perimeter" means. It means the distance all the way around all the sides of the drawing.
The sides of this drawing are 12a, 6a+8, 10a-4, and 6a+8 .
The perimeter is the sum of all four lengths, so it's
(12a) + (6a+8) + (10a-4) + (6a+8) .
Clean that up and combine the like terms, (add up all the 'a's
and add up all the plain numbers), you'll have the expression
for the perimeter.
Then, once you have the expression, set it equal to 250,
and you'll have an equation that you can solve to find ' a '.
Doing it in my head, I got a=7. I don't know if that's correct.
