Let f(x)=12x\displaystyle {f{{\left({x}\right)}}}=\sqrt{{{12}-{x}}}. Compute f(8)\displaystyle {f}'{\left({8}\right)} using the limit definition

f(8)\displaystyle {f}'{\left({8}\right)} =  

Find an equation of the tangent line at x=8\displaystyle {x}={8}

y\displaystyle {y} =