TeX source:

x, m_{\tan }=\lim\limits_{h \rightarrow 0} \frac{f(x+h)-f(x)}{h}=\lim\limits_{h \rightarrow 0} \frac{7(x+h)-7 x}{h}=\lim\limits_{h \rightarrow 0} \frac{7 h}{h}=\lim\limits_{h \rightarrow 0} 7=7