The line x=1 passing through the vertex of parabola is the line of symmetry.
Symmetrical to x=3 in respect to x=1 is x=-1,
so f(3) = f(-1), b=-1.
In a "normal" language.
f(3)=5 (see the graph), there are two points with y=5: (3,5) and (-1,5) (both marked!),
f(-1) = 5 and b=-1