To find the length \( x \) of the garden, we can solve the equation given:
\[ x(x - 4) = 140 \]
First, expand the left side:
\[ x^2 - 4x = 140 \]
Next, move all terms to one side of the equation to set it to zero:
\[ x^2 - 4x - 140 = 0 \]
Now we can use the quadratic formula to solve for \( x \). The quadratic formula is:
\[ x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \]
In our equation, \( a = 1 \), \( b = -4 \), and \( c = -140 \).
We calculate the discriminant:
\[ b^2 - 4ac = (-4)^2 - 4(1)(-140) = 16 + 560 = 576 \]
Now plug the values into the quadratic formula:
\[ x = \frac{-(-4) \pm \sqrt{576}}{2(1)} \] \[ x = \frac{4 \pm 24}{2} \]
This gives us two possible solutions for \( x \):
- \( x = \frac{4 + 24}{2} = \frac{28}{2} = 14 \)
- \( x = \frac{4 - 24}{2} = \frac{-20}{2} = -10 \)
Since a length cannot be negative, we discard \( -10 \) as a solution.
Thus, the length \( x \) of the garden is:
\[ \boxed{14} \text{ feet.} \]