I have to find a cubic function, when I know the gradient of that line at its start and finish.
The gradient at the start is 0.2, and at the top -0.4.
Using the cubic function: $f(x)=ax^3+bx^2+cx+d$ I need to model the section in between the red lines. Basically I assume I need to find $a,b,c,d$ and use the derivative of the cubic function with the gradients somehow (the gradients on the diagram are the gradients at the end of the red lines).
Thanks!