Simplified Lagrange interpolation formula inside Shamir's secret sharing scheme
All these calculations are done $\bmod{17}$.

$2^{-1}$ means here the number, $x$, that satisfies the equation $2x = 1 \pmod{17}$. (Note that this is the same definition that we use for the reals). When you're working $\bmod{17}$ (or any prime number, in fact), every number except 0 has an inverse.

To calculate inverses efficiently, you should use the extended Euclidean algorithm. But with a small number like 17, it might be worthwhile to just construct a multiplication table to get practice with modular arithmetic.