Pls help I am stuck and I don't know what to do

Maybe c=0 or ax^2+bx=-c?

But remember....when f(0)    x = 0    so you get    a(0)^2 + 0 * x  + c       and this equals 'c'

                                                                                   AND you are told  f(0) = 0                        NOW what can you say about 'c' ?

Do you understand what EP is telling you ?

Yes, yu wre right the first time.  c=0

so now you have    

$f(x)=ax^2+bx\\ now \;what?$

sub in the other 2 and you will get two equations that you can solve simultaneously to get a and b.

Give it a go and get back to us with what you have tried