Jogger Vs. The Oak Tree

If a healthy but somewhat direction impaired individual (mass = 81.9 kg) is jogging through the woods and runs straight into a large oak tree at 4.7 m/s. Rebound speed is measured at 4.2 m/s in the opposite direction. If the time of contact with the tree is 46 milliseconds, what is the magnitude of the force that the tree exerts on the jogger?

What have you done? I suggest applying the acceleration definition ($a = \frac{\Delta v}{\Delta t}$) directly into Newton's 2nd Law.