Finding the Derivative by the Limit Process In Exercises $15-28,$ find the derivative of the function by the limit process.

$15.f(x)=7$

$16.g(x)=-3$

$17.f(x)=-5x$

$18.f(x)=7x-3$

$19.h(s)=3+\frac{2}{3}s$

$20.f(x)=5-\frac{2}{3}x$

$21.f(x)=x^2+x-3$

$22.f(x)=x^2-5$

$23.f(x)=x^3-12x$

$24.g(t)=t^3+4t$

$25.f(x)=\frac{1}{x-1}$

$26.f(x)=\frac{1}{x^2}$

$27.f(x)=\sqrt[\phantom{2}]{x+4}$

$28.h(s)=-2\sqrt[\phantom{2}]{s}$