Sort the data points into scending order for x.
Put the values for the first third and fifth data points into this which will give you a set of three simultaneous equations for this, which you can solve for a,b and c.
Alternativly look up quadratic regression on Google.
If you don't want a quadratic fit you can repeat the first method for a cubic or quartic.
It would also help if you provided more background information on the source of this question.