Question

a) 20+{[7+5+(-9-7)+3]}

Answer 1

20+{[12-16+3]}=
20-1=
19

Answer 2

$20 + \{ \ [ 7 + 5 + (-9-7) + 3\ ] \} \\ \\ \\ 20 + \{ \ [ 12 + (-16) + 3\ ] \} \\ \\ \\ 20 + \{ \ [ 12 - 13 \ ] \} \\ \\ \\ 20 + \ -1 => 19$