I don't quite understand how to derive from first principals.

I understand that it is a point x+h - x for delta x and how h is delta y but i still have trouble working through them. i have tried googleing a few examples but the formatting hurts my brain.

Would anyone mind going through an example for me of 3x^2+4x-6

I believe it starts at (3(x+h)^2+4(x+h)-6)-(3x^2+4x-6) all over h lim as h -> 0 but would someone mind taking me step by step through it?