Suppose your average after taking 5 quizzes is 70 out of 100. What mud your average be on the next 5 quizzes to increase your average to 85 out of 10”?

Let
x-----------> the width of the wall
x+15------> the length of the wall 

we know that
x*(x+15)+3*[3*4] <=202 ft²
x²+15x+36 <=202
x²+15x+36 -202<=0
x²+15x-166<=0

the answer is 
x²+15x-166<=0

using a graph tool
see the attached figure
The maximum value of x is x=7.408 ft