Idiomatic C would say:

  float discrim = b*b - 4*a*c;

Using pow for a small integer power is a no-no: less efficient and less accurate.

I agree that \ is an awkward choice. A Fortran-like double asterisk ∗∗ is out because of ambiguity with pointers; single caret ^ out because it is already reserved for bitwise xor. Maybe double caret ^^ or asterisk-caret ∗^ could be used? That would read okay :

  double discrim = b^^2 - 4*a*c;
  double discrim = b*^2 - 4*a*c;