If a rubber band is stretched a particular distance, the restoring force is defined as the force needed to
restore the rubber band to its original form. When a
particular rubber band is stretched to a length of 30
centimeters, the restoring force is –35 newtons, and
when it is stretched to a length of 90 centimeters, the
restoring force is –125 newtons. For every additional
20 centimeters from its original form, the restoring
force decreases by c newtons, where c is a constant.
What is the value of c ?