Exercise $4\mathrm{.}2\mathrm{.}4$: Which of the following identities are true? b$)$

a$($a$\backslash$L$)=$L

$($again$,$ concatenation with

$\{$a$\}$

$,$ this time on the left, is intended$).$ c$)$

$($La$)/$a$=$L