We don't know that at all. The 3rd law says that the REaction is opposite and EQUAL to the action. We don't know where that "twice as much" comes from.
The length of the string must vary, while the angle and acceleration are constant
PV=nRT
(720/760)(0.200)=(0.800/x)(0.08206)(323.15)
(0.1894736842)=(0.800/x)(0.08206)(323.15)
.0071451809=(0.800/x)
x=MM=111.9635758 g/mol
Answer:
x₂=0.44m
Explanation:
First, we calculate the length the spring is stretch when the first block is hung from it:

Now, since the stretched spring is in equilibrium, we have that the spring restoring force must be equal to the weight of the block:

Solving for the spring constant k, we get:

Next, we use the same relationship, but for the second block, to find the value of the stretched length:

Finally, we sum this to the unstretched length to obtain the length of the spring:

In words, the length of the spring when the second block is hung from it, is 0.44m.