Final answer:
The value of x for which the area within the fence is equal to 0 is x = -1.
Step-by-step explanation:
To find the value of x for which the area within the fence is equal to 0, we need to solve the quadratic equation 2x² - 11x - 21 = 0. We can factorize the equation or use the quadratic formula to find the solutions. The solutions are x = 7 and x = -1. Therefore, the correct answer is C) x = -1.