Answer
a) See the graph below.
b) at $t=1\;\rm s $
Work Step by Step
a) We have 3 stages of motion. And we know that the velocity of each stage is given by the slope of an $x-t$ graph.
So,
$\Rightarrow$ the velocity of the first stage which lasts for 1 second is
$$v_1=\dfrac{x_2-x_1}{t_2-t_1}=\dfrac{20-0}{1-0}=\color{red}{\bf20} \;\rm m/s$$
$\Rightarrow$ the velocity of the second stage which lasts for 2 seconds is
$$v_2 =\dfrac{10-20}{3-1}=\color{red}{\bf -5}\;\rm m/s$$
$\Rightarrow$ the velocity of the third stage which lasts for 1 second is
$$v_3 =\dfrac{10-10}{4-3}=\color{red}{\bf 0}\;\rm m/s$$
Now we can easily draw the graph, as you see below.
b)
There is one turn point which is at $t=1\;\rm s$.
You can notice that the velocity was positive before $t=1\;\rm s$ and then became negative after that. This is a turning point.
