This one is based on that all-important fact: parabolas are symmetrical! When they say f(b) = f(3), that's another way of saying that you have two points, (b,y) and (3,y), and the y-values are the same. So look at the graph, and see what that y-value is!
On the graph, I'm seeing the point (3,5), so our other point must be (b,5). The only other place this graph goes through y=5 is at (-1,5), so b must equal -1.