What is the extremum value of line y equals mx plus c?

Answer 1

A straight line, defined on an unrestricted domain, has no extrema.

If m>0 then, as x goes from - infinity to + infinity, then so does y, while

if m<0 then, as x goes from - infinity to + infinity, y goes from + infinity to - infinity. There are NO turning points.

In the degenerate case that m = 0, then y = c for all values of x and that could be considered an extremum.