Yes the text in the red is correct.
Answer:
In words, it could be said that the force times the time equals the mass times the change in velocity. In physics, the quantity Force • time is known as impulse. And since the quantity m•v is the momentum, the quantity m•Δv must be the change in momentum. The equation really says that the Impulse = Change in momentum.

Explanation:
Natural length of a spring is
. The spring is streched by
. The resultant energy of the spring is
.
The potential energy of an ideal spring with spring constant
and elongation
is given by
.
So, in the current problem, the natural length of the spring is not required to find the spring constant
.

∴ The spring constant of the spring = 
Answer:
0.102 m
Explanation:
k = spring constant of the spring = 125 N/m
m = mass of the block attached to the spring = 650 g = 0.650 kg
x = maximum extension of the spring
h = height dropped by the block = x
Using conservation of energy
Spring potential energy gained = Gravitational potential energy lost
(0.5) k x² = mgh
(0.5) k x² = mgx
(0.5) (125) x = (0.650) (9.8)
x = 0.102 m