3 + { 4 + 2 × [ 2 + ( 6 + a ) : 3 ] - 17 } = 24
4 + 2 × [ 2 + ( 6 + a ) : 3 ] - 17 = 24 - 3
4 + 2 × [ 2 + ( 6 + a ) : 3 ] - 17 = 21
4 + 2 × [ 2 + ( 6 + a ) : 3 ] = 21 + 17
4 + 2 × [ 2 + ( 6 + a ) : 3 ] = 38
2 × [ 2 + ( 6 + a ) : 3 ] = 38 - 4
2 × [ 2 + ( 6 + a ) : 3 ] = 34
2 + ( 6 + a ) : 3 = 34 : 2
2 + ( 6 + a ) : 3 = 17
( 6 + a ) : 3 = 17 - 2
( 6 + a ) : 3 = 15
6 + a = 15 × 3
6 + a = 45
a = 45 - 6
a = 39
