Solution:

Superposition: Systems that satisfy both homogeneity and additivity are considered to be linear systems. These two rules, taken together, are often referred to as the principle of superposition.
Homogeneity: As we increase the strength of a simple input to a linear system, say we double it, then we predict that the output function will also be doubled.
If the system is linear, then the measured response will be just the sum of its responses to each of the two stimuli presented separately.