Question 330282
Assuming length = a, and width = b = a-4.
Area = a*b = 12
{{{a*(a-4) = 12}}}
{{{a^2 - 4a = 12}}}
{{{a^2 - 4a - 12 = 0}}}
{{{(a+2)(a-6) = 0}}}

a + 2 = 0
a = -2; not impossible.

a - 6 = 0
a = 6

If a = 6, then
a*b = 12
6b = 12
b = 12/6 = 2

Hence, length = 6 feet and width = 2 feet.