a)27 - (5 - |a|) = 31
=>5 - |a|=27-31
=>5 - |a|=-4
=>|a|=5 -(-4)
=>|a|=9
=>a=±9
b) -13 - (6 - |a + 1|) = 24
=>6 - |a + 1|=-13-24
=>6 - |a + 1|=-37
=>|a + 1|=6 -(-37)
=>|a + 1|=43
=>\(\left[ \begin{array}{l}a+1=43\\a+1=-43\end{array} \right.\)
=>\(\left[ \begin{array}{l}a=43-1\\a=-43-1\end{array} \right.\)
=>\(\left[ \begin{array}{l}a=42\\a=-44\end{array} \right.\)
vậy a∈{42;-44}