Answer
a) In phase.
b) See the table below.
Work Step by Step
a) We can see, from the given figure, that the waves of the two sources are at the same distance from their source at the same time interval.
Thus, they are in phase.
b) From the given graph, we can find $r_1 $ for each point which is the distance from the first source, $r_2$ which is the distance from the second source, $\Delta r=|r_2-r_1|$ which is the path length difference, $\rm C$ or $\rm D$ which is constructive or destructive.
\begin{array}{|c|c|c|c|}
\hline
\rm points&r_1 &r_2& \Delta r&\rm C/D\\
\hline
\rm P& 3\lambda & 4\lambda&\lambda& \rm C\\
\hline
\rm Q & 3.5\lambda & 2\lambda &1.5\lambda&\rm D\\
\hline
\rm R& 2.5\lambda & 3.5\lambda&\lambda&\rm C \\
\hline
\end{array}