Final answer:
To find the value of x, use the formula for the perimeter of a rectangle and solve for x. The value of x is 5 cm, corresponding to option A).
Step-by-step explanation:
To find the value of x, we need to use the formula for the perimeter of a rectangle. The formula is P = 2w + 2l, where P is the perimeter, w is the width, and l is the length. In this case, we are given that the perimeter is 40 and the length is 3x. Substituting these values into the formula, we get 40 = 2x + 2(3x). Simplifying the equation, we have 40 = 2x + 6x. Combining like terms, we get 40 = 8x. Dividing both sides of the equation by 8, we find that x = 5. Therefore, the value of x is 5 cm, which corresponds to option A).