The definition of a derivative is
f ′ ( x ) = lim h → 0 f ( x + h ) − f ( x ) h {\displaystyle \displaystyle f'(x)=\lim _{h\to 0}{\frac {f(x+h)-f(x)}{h}}}
Alternatively, one can use
f ′ ( a ) = lim x → a f ( x ) − f ( a ) x − a {\displaystyle \displaystyle f'(a)=\lim _{x\to a}{\frac {f(x)-f(a)}{x-a}}}