If on the graph you have a b c on the positive side of the x-axis and you you have one point located above b at approximately (b.3),
l y
l >
l .
l<
l--------------x>
a b c
now if you draw a line from< to the . to the > it looks like an s.
The question is: f'(x)>0 on (a,c); f"(x)<0 on (a,b) and f"(x)>0 on (b,c)
l y
l >
l .
l<
l--------------x>
a b c
now if you draw a line from< to the . to the > it looks like an s.
The question is: f'(x)>0 on (a,c); f"(x)<0 on (a,b) and f"(x)>0 on (b,c)