Problem $2$: Find a general formula $($if possible$)$ for the Taylor series centered at $a,$ of the following functions:

a$.f(x)=\frac{1}{x},a=1$

b$.f(x)=\sqrt[\phantom{2}]{x},a=4$

c$.f(x)=x^4+3x-1,a=2$

d$.f(x)=e^{3x},a=-1$