If you replace a by x, you can make it change from 0 to width (of the sketch) for example, ie. the values represent the coordinates on the x axis.
By computing the formula for each x value, you get the corresponding y values.
You can then draw a line between the newly computed (x, y) and the previous one.

type "2x^2 + 2x" into google and you'll see what the problem is - the graph only makes sense for small positive and negative values of x and yours get into the hundreds. use smaller x increments and scale them appropriately.

(plus yours is upside down as processing has 0, 0 at top left of screen...)