1000-[36-6X9-(a:3+15)]:3=76
1000 - [36 - 54-(a:3+15)] : 3 = 76
[36 - 54 - (a:3+15)] : 3 = 1000 - 76
[36 - 54 - (a:3+15)] : 3 = 924
36 - 54 - (a:3+15) = 924×3
36 - 54 - (a:3+15) = 2 772
a : 3 + 15 = 36 - 54 - 2772
a : 3 = - 2790 - 15
a : 3 = - 2805
a = -2805 × 3
a = - 8415