Answer:
K = 26.667 N/m
Step-by-step explanation:
a) Using the newton's laws:

Where W is the weigth of the block in the second spring and
is the force of that spring. So:
W =

It means that:
W =

where K is the constant of the spring and x2 is the deformation of the second spring. Then, replacing values we get:
4N = K(0.15m)
Solving for K:
K = 26.667 N/m
b) At the same way, for the third spring, we have the equation:
W =

Where K is the same constant of the secor spring and x3 is the deformation of the third spring.
Finally, replacing values, we get:
W = (26.667N/m)(0.30)
W = 8N