Doug C. answered 06/08/20
Math Tutor with Reputation to make difficult concepts understandable
f(x)= -19 (constant function--no matter the input the output is -19)
So,
f(x+h) = -19
f(x) = -19
f(x+h) - f(x) = -19 - (-19) = 0
[f(x+h) - f(x)] / h = 0/h = 0
limh->0 0 = 0 (limit of a constant = constant).