The 2/8 at the end seems odd, are you sure the whole thing isn't divided by 8? I'll work it as it's shown
2[12 - 18] = 2*12 - 12*18 = 24 - 36 = -12
2/8 = 1/4
So
2[12 - 18] - 2/8 = -12 - 1/4
To do the subtraction, you need the 12 to also be a fraction with 4 on the bottom, so
-12 - 1/4 = -48/4 - 1/4 = -49/4