Answer
$a-\,\,\\
A\bigtriangleup B=\left \{ 1,2,5,6 \right \}\\$
$b-\,\,\\
B\bigtriangleup C= \left \{ 3,4,7,8 \right \}\\$
$c-\,\,\\
A\bigtriangleup C=\left \{ 1,2,3,4,5,6,7,8 \right \}\\$
$d-\,\,\\
(A\bigtriangleup B)\bigtriangleup C=
\left \{ 1,2,7,8 \right \}$
Work Step by Step
$A=\left \{ 1,2,3,4 \right \},B=\left \{ 3,4,5,6 \right \},C=\left \{ 5,6,7,8 \right \}\\
A-B=\left \{ 1,2 \right \},B-A=\left \{ 5,6 \right \}\\
A\bigtriangleup B= (A - B) \cup (B - A) \\
A\bigtriangleup B=\left \{ 1,2 \right \}\cup \left \{ 5,6 \right \}=\left \{ 1,2,5,6 \right \}\\
B-C=\left \{3,4 \right \},C-B=\left \{ 7,8 \right \}\\
B\bigtriangleup C= (B - C) \cup (C - B) \\
B\bigtriangleup C= \left \{ 3,4 \right \}\cup \left \{ 7,8 \right \}=\left \{ 3,4,7,8 \right \}\\
A-C=\left \{1,2,3,4 \right \},C-A=\left \{ 5,6,7,8 \right \}\\
A\bigtriangleup C= (A - C) \cup (C - A) \\
A\bigtriangleup C= \left \{ 1,2,3,4 \right \}\cup \left \{ 5,6,7,8 \right \}\\
A\bigtriangleup C=\left \{ 1,2,3,4,5,6,7,8 \right \}\\
$
$(A\bigtriangleup B)\bigtriangleup C=((A\bigtriangleup B)- C)\cup (C-(A\bigtriangleup B))\\
A\bigtriangleup B=\left \{ 1,2,5,6 \right \},C=\left \{ 5,6,7,8 \right \}\\
(A\bigtriangleup B)- C =\left \{ 1,2 \right \}\\
C-(A\bigtriangleup B)=\left \{ 7,8 \right \}\\
(A\bigtriangleup B)\bigtriangleup C=((A\bigtriangleup B)- C)\cup (C-(A\bigtriangleup B))\\
(A\bigtriangleup B)\bigtriangleup C=\left \{ 1,2 \right \}\cup \left \{ 7,8 \right \}=
\left \{ 1,2,7,8 \right \}$