you got "c" by substituting in x=0 and y=2, as y=f(x).
You need "a" and "b" so you need two clues.
You've been given the clues.
If you now use that value of "c", you can use the second point given,
as it gives
If you differentiate the function, you get the gradient,
hence that is your final clue.
You will have a pair of simultaneous equations to solve for "a" and "b".