6 + [ 4 - 4 × ( 4 - 4 : 4 × 4 - 4 ) + a ] : 4 = 10
6 + [ 4 - 4 × ( 4 - 1 × 4 - 4 ) + a ] : 4 = 10
6 + [ 4 - 4 × ( 4 - 4 - 4 ) + a ] : 4 = 10
6 + [ 4 - 4 × ( 0 - 4 ) + a ] : 4 = 10
6 + [ 4 - 4 × ( - 4 ) + a ] : 4 = 10
6 + [ 4 - ( - 16 ) + a ] : 4 = 10
6 + ( 20 + a ) : 4 = 10
( 20 + a ) : 4 = 10 - 6
( 20 + a ) : 4 = 4
20 + a = 4 × 4
20 + a = 16
a = 16 - 20
a = - 4
