[( 9 × a - 21 ) : 3 + 75] :7 = 358 - 74 × 4
[( 9 × a - 21 ) : 3 + 75] : 7 = 358 - 296
[( 9 × a - 21 ) : 3 + 75] : 7 = 62 / ×7
[( 9 × a - 21 ) : 3 + 75] = 434
( 9 × a - 21 ) : 3 = 434 - 75
( 9 × a - 21 ) : 3 = 359
( 9 × a - 21 ) = 359 × 3
( 9 × a - 21 ) = 1.077
9 × a = 1077 + 21
9 × a = 1.098
a = 1098 : 9
a = 122
