K7ITM wrote:
"If you say that in the system (y = mx + b) that b is the response to
zero input (x=0) then you will conclude the system is nonlinear."

Why? the factor (b) is a constant, a value to be added to (mx) to total
a value for (y).

(mx) is a straight line. Every value of (b) produces a straight lline
parallel with lhe line y=mx when b=0. Factor (b) is merely the offset
value of the sloped line in the x direction.

y=mx+b is listed in math books as a defining example of a linear
equation. (When plotted, a linear equation produces a straight line.)
y=mx+b has a special name: "The point slope formula". Perfectly
descriptive, too.

To clarify everything, graph a few values for yourself.

Best regards, Richard Harrison, KB5WZI