The function is a line, so it's derivative is a fixed constant.
the domain is all reals
f(x+h) = m(x+h) + n = mx + mh + n
f(x+h) - f(x) = mx + mh + n - mx - n = mh
dividing by h gives a difference quotient of just m
the limit as h tends to zero does not change anything.
the derivative is m, the slope of the line