a
(a) Retarding force $F = ma = \mu \,R = \mu \;mg$
$\therefore a = \mu g$
Now from equation of motion ${v^2} = {u^2} - 2as$
$ \Rightarrow \;0 = {u^2} - 2as$
$⇒$ $s = \frac{{{u^2}}}{{2a}} = \frac{{{u^2}}}{{2\mu \;g}}$
$\therefore s= \frac{{v_0^2}}{{2\mu \,g}}$