[tex]\bf f(t)=14-5t\qquad \qquad \stackrel{d e f in i tion~of~a~derivative}{\lim\limits_{h\to 0}~\cfrac{f(t+h)-f(t)}{h}}
\\\\\\
\lim\limits_{h\to 0}~\cfrac{[14-5(t+h)]~~-~~[14-5t]}{h}\implies \lim\limits_{h\to 0}~\cfrac{\underline{14-5t}-5h\underline{-14+5t}}{h}
\\\\\\
\lim\limits_{h\to 0}~\cfrac{-5\underline{h}}{\underline{h}}\implies -5[/tex]
