[6×(12:x+56):3-18]×7+78=792
[6×(12:x+56):3-18]×7 = 792-78
[6×(12:x+56):3-18]×7 = 714
[6×(12:x+56):3-18] = 714 : 7
6×(12:x+56):3-18 = 102
6×(12:x+56):3 = 102+18
6×(12:x+56):3 = 120
6×(12:X+56) = 120×3
6×(12:X+56) = 360
12:X+56 = 360 : 6
12:X+56 = 60
12:X = 60-56
12:X = 4
X = 12:4
X = 3