F(x)= 2(4)-12
F(x)= 8-12
-4 is the answer
Perimeter Formula:

Area Formula:

Set perimeter Formula equal to b:

Plug formula for be we just made above into the area Formula:

Solve for L (you'll have to use quadratic formula) then plug that back into the area Equation and solve for b.
Here's a link to where the problem has been explained before (they go into more detail than me).
https://brainly.ph/question/468631
lmk if you need any more help
The answer is 47. Just keep adding 4.