$[16]2.$ For the function $f(x)=6\mathrm{.}5x-x^2,$ do the following:

a$)$ Use definition $(1)$ or definition $(2)$ from section $3\mathrm{.}1$ to find the value of $f^{\text{'}}(2).$

b$)$ Use the definition of the derivative from textbook section $3\mathrm{.}2$ to find $f^{\text{'}}(x).$

c$)$ Find the equation of the line which is tangent to the graph of $f$ at the point where $x=4.$