Feet per mile is not a linear equation. The mile is the X and the 1714 ft is the Y. If it has not been solved yet, give me a few minutes.
Given your 79s estimation, the speed the object travels, on a lateral plane, is 318.987 mph. The problem is the fact that 318 mph is a mean number and it likely only traveled that speed once. The speed of the object is much slower at miles 0-2 than it is from 4-7. Also, this problem was impossible without time.