Yes i have learned about differential equation, and i know how to solve it but how do i form a function where to apply it? I also know the formula for a slope of a line, what usually forms from x, but here slope forms from y.Sounds like a differential equation to me. What have you learned about them? (I assume you know what an abscissa is.)
Even if you haven't heard the term "differential equation", you should be able to write an equation describing this situation.