pt2pt1 =
@ (M) (((y + 1) * M ^ 2) / ((y-1) * M ^ 2 + 2)) ^ (y / (y-1)) * ((y + 1) / (2 * y * M ^ 2 - (y-1))) ^ (1 / (y-1)) -.6925
fsolve (pt2pt1, 2)