SoapSimThat graph is obtained by:
Solve this for v: http://blog.wolfram.com/data/uploads/2012/10/Speed-Equation31.png
Then divide by the speed of sound at that altitude:
http://blog.wolfram.com/data/uploads/2012/10/Speed-Out5.png
You're not going to be able to find the "correct" equation, but the graph somewhat resembles a (negative) quartic graph so you could stick coordinates on the graph into a system of equations to get coefficients. Maybe.
While this method might work. I think he needs to find a way that works for his 12th grade functions class.
Not really feasible for him to submit something like that on a test.
youre right, it has to be simple enough so that i can present in a way that my class can understand it, im going to consult with my teacher and see if i can model it as a combination of functions and if not ill just model it as a cubic function even if its not perfect.