The answer is true.
Point X lies on the line of intersection of planes S and R. This means that point X lies on both plane S and plane R at the same time.
You could also try and imagine the diagram without plane S and see that point X lies on plane R. Then you can imagine the same diagram without plane R and see that point X lies on plane S.
Let me know if you need any clarifications, thanks!
~ Padoru
