Step-by-step explanation:
you mean "differentiable" ?
well, remember, a line equation is
y = ax + b
a is the slope
it is always, ALWAYS the factor of x.
now we differentiate the line equation per dx/dy
y' = a
and that is that.
the first derivative of an equation of first degree (that means the highest exponent of the variable is 1), which is a line equation, is a constant : the factor of x.
and since the factor of x is the slope, that means that the constant result of the first derivative of the line equation is the slope.