( a × 5 + 28 - 9 ) × 2 + 75 - 46 = 47
( a × 5 + 28 - 9 ) × 2 + 75 = 47 + 46
( a × 5 + 28 - 9 ) × 2 + 75 = 93
( a × 5 + 28 - 9 ) × 2 = 93 - 75
( a × 5 + 28 - 9 ) × 2 = 18
a × 5 + 28 - 9 = 18 : 2
a × 5 + 28 - 9 = 9
a × 5 + 28 = 9 + 9
a × 5 + 28 = 18
a × 5 = 18 - 28
a × 5 = - 10
a = - 10 : 5
a = - 2