we know that
step 1
Find the surface area of the box
the surface area of the box is equal to
S=2*area of the base+perimeter of the base*height
in this problem
area of the base=11*3=33 in^2
perimeter of the base=2*(11+3)--------> 28 in
height=x
S=2*33+28*x--------> S=(66+28*x) in^2
step 2
Find the Volume of the box
the volume of the box is equal to
Volume=area of the base*height
area of the base=11*3=33 in^2
height=x
Volume=33*x in^3
step 3
equate the value of the surface area with the value of the volume
(66+28*x)=33*x
33*x-28*x=66
5*x=66
x=66/5-------> x=13.2 in
the answer is
the value of x is 13.2 in