For the$$ function,

$$f$($x$)$equals$=$StartRoot $21$ minus x EndRoot$21$x$,$

find the following.

$($a$)$

$$ f$(5)$

$($b$)$

$$f$($negative $8)$

$($c$)$

$$f$(10\mathrm{.}2)$

$($d$)$

$$f$($negative $6\mathrm{.}4)$