Use the first point , 0,5 to find the value of 'c'
then use the second set of points to find an equation with 'a' and 'b' in it
use the third set of points to find ANOTHER equation with 'a' and 'b' in it
then use the two equations to solve for 'a' and 'b' via elimination or substitution.
Re-post if you need more help.... Go !