hi all, i need some help on finding quadratic equations from points on graphs, could someone please please explain how i would determine which general equation to use to solve.

1.) A parabola has a y intercept of 3 and x intercepts of 1 and 3, find the rule dictating the parabola.

Mr F says: Use the model . B = 1, C = 3 (why?). To get A substitute the given y-intercept, that is, the point (0, 3).
a.) someone please explain how to pick a formula to use

2.) a parabola passes through points (-3,9) find the rule dictating the parabola.

Mr F says: Insufficient data to get a unique rule.
a.) someone please explain how to pick a formula to use

also at the start of the exercise there is an example like this: a parabola passes through points (0,3) and (-3,1) it says this equation is in the from y =ax^2 + c... What the hell? i nvr learnt any ax^2 +c

Mr F says: They're giving you the heads-up that b = 0 in the standard form! Run with the ball!! Substitute the two given points to get two equations in a and c. Solve these two equations simultaneously.
pls help i have a test this coming thursday