A quadratic formula implies calculating position from time, given some acceleration and starting velocity. The delta doesn't even need to be calculated in this case, you simply need to insert the time into the formula and you'll have the current position.
The delta does matter if, say, you are accelerating at a variable rate, or some other such task.
However, if you really want to know, to find the delta you need to take the derivative of the formula, or better yet, a discrete derivative. The derivative of a quadratic, given a*x^2+b*x+c is 2a*x+b. That should be your delta.