[( 5 + a ÷ 3 ) ÷ 13 + 8 ] × 6 - 12 = 42
[( 5 + a ÷ 3 ) ÷ 13 + 8 ] × 6 = 42 + 12
[( 5 + a ÷ 3 ) ÷ 13 + 8 ] × 6 = 54
( 5 + a ÷ 3 ) ÷ 13 + 8 = 54 : 6
( 5 + a ÷ 3 ) ÷ 13 + 8 = 9
( 5 + a ÷ 3 ) ÷ 13 = 9 - 8
( 5 + a ÷ 3 ) ÷ 13 = 1
5 + a ÷ 3 = 1 × 13
5 + a ÷ 3 = 13
a ÷ 3 = 13 - 5
a ÷ 3 = 8
a = 8 × 3
a = 24