remember that the axis of symetry is x=value of how far it is from the origin
also remember that the vertex formula is -b/(2a) so
a=3
3/2=-b/(2(3))
3/2=-b/(6)
mutiply both sides by 6
18/2=-b
9=-b
multily both sides by -1
-9=b
so the equation is
f(x)=3x^2-9x+4
b=-9