Compute the each part of given problem:

Problem: The Achilles tendon, which connects the calf muscles to the heel, is the thickest and strongest tendon in the body. In extreme activities, such as sprinting, it can be subjected to forces as high as 12.0 times a person's weight. According to one set of experiments, the average area of the Achilles tendon is 78.7 mm2, its average length is 20cm , and its average Young's modulus is 1474MPa.

Part I: How much tensile stress is required to stretch this muscle by 5.4% of its length?

Part II: If we model the tendon as a spring, what is its force constant?

Part III: If a 75kg sprinter exerts a force of 12.0 times his weight on his Achilles tendon, by how much will it stretch?

Explain the each and every step in derail for help.