6+[15-(8:a×7+7)]×5=8:(1×4)+3×3
6+[15-(8:a×7+7)×5=8:4+9
6+[15-(8:a×7+7)×5=2+9
6+[15-(8:a×7+7)×5=11
[15-(8:a×7+7)]×5=11-6
[15-(8:a×7+7)]×5=5
15-(8:a×7+7)=5:5
15-(8:a×7+7)=1
-(8:a×7+7)=1-15
-8:a×7-7=-14
-8:a×7=-14+7
-8:a×7=-7
-56:a=-7 |:(-1)
56:a=7
a=56:7
a=8
