Use parametrized equation

[sverhoeven]

At the moment the equation (and its derivative) is hardcoded to 2x^3 - 4x^2 + 6 as a user I would like to use a different equation or change the multipliers. So for example

float equation(float x) {
  return a * x * x * x - b * x * x + c;
}

Where 'a','b' and 'c' can be configured from a submit form.