A9.0-cm-long spring is attached to the ceiling. when a 1.8 kg mass is hung from it, the spring stretches to a length of 18 cm . you may want to review ( pages 219 - 221) . part a what is the spring constant k?

The spring constant is computed by:
F = kx

Where: F is the force applied in newtons (N)

k is the spring constant measured in newtons per meter (N/m); and

x is the distance the spring is stretched (m)
and

F = mg

Where: F is the force pulling objects in the direction of the Earth.

m is the mass of the object.

g is the acceleration due to gravity; 
So plugging our values in the formula:

F = mg

 = (1.8) (9.81) = 17.658N 

k =

F/x = 17.658 /0.09 = 196.2 N/meter