Can someone pls show me the full working out for this question? I did not understand how to form the equation to solve for the unknown value on the tree diagram
I will add the first part of the question as information there is relevant to yours:
It should be easy to draw a probability tree based on this information. The part above just adds another level to the probability tree  students saving or not saving water.
Two key pieces of information were given in the question:

Students who were concerned about water shortages (W) were twice as likely to save water (S) than those who were not concerned about water shortages (W’). We will then assign x to the probability that a student from the group that was not concerned about water shortages (W’) saved water (S) and 1x to the probability that a student from the group W’ didn’t save water (S’). Then I will assign 2x to the probability that a student from the group which is concerned about water shortages (W) saved water (S) and 12x to the probability that a student from the group W didn’t save water (S’).

Concern over climate change didn’t effect probability of students saving water, so we will assign the same x, 1x, 2x, and 12x to the group which was not concerned about climate change.
Now we can create an equation adding all probabilities that a student saved water:
0.7 (0.85 * 2x +0.15x) + 0.3(0.55 * 2x + 0.45x) = 0.5632
Solving the equation for x we will have x = 0.32
Then to answer the question:
P(C’ and W’ and S’) = 0.3 * 0.45 * (1  0.32) = 0.0918