function hornerMethod(coefficients, xVal) {
  return coefficients.reduce(
    (accumulator, currentCoefficient) => {
      return accumulator * xVal + currentCoefficient;
    },
    0,
  );
}
Returns the evaluation of a polynomial function at a certain point.
Uses Horner's rule.

@param {number[]} coefficients - i.e. [4, 3, 2] for (4 * x^2 + 3 * x + 2)
@param {number} xVal
@return {number}