Answer
14
Work Step by Step
$k'(z)=\lim\limits_{h \to 0}\frac{k(z+h)-k(z)}{h}=\lim\limits_{h \to 0}\frac{[14(z+h)+12]-[14z+12]}{h}$
$=\lim\limits_{h \to 0}\frac{14z+14h+12-14z-12}{h}=14$
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