Answer
$A\bigtriangleup \varnothing = (A - \varnothing ) \cup (\varnothing - A) \\
=(A \cap \varnothing^c ) \cup (\varnothing \cap A^c) \\
(set\,\,dif\! ference\,\,law )\\
=(A \cap U ) \cup (\varnothing \cap A^c) \\
(complement\,\,law\,for\,\varnothing) \\
=A\cup (\varnothing \cap A^c)\\
(identity\,law\,for\,\cap )\\
=A\cup \varnothing \\
(def.of\varnothing )\\
=A
$
Work Step by Step
$A\bigtriangleup \varnothing = (A - \varnothing ) \cup (\varnothing - A) \\
=(A \cap \varnothing^c ) \cup (\varnothing \cap A^c) \\
(set\,\,dif\! ference\,\,law )\\
=(A \cap U ) \cup (\varnothing \cap A^c) \\
(complement\,\,law\,for\,\varnothing) \\
=A\cup (\varnothing \cap A^c)\\
(identity\,law\,for\,\cap )\\
=A\cup \varnothing \\
(def.of\varnothing )\\
=A
$