To solve this equation, we will first simplify each side separately.

1/2 + 1/4 - 1/5 = 9/20

x - 3 + 3 - 456 + 56 = x - 400

Now, we have the equation:

9/20 = x - 400

To solve for x, we will add 400 to both sides of the equation:

9/20 + 400 = x

x = 400 + 9/20
x = 8000/20 + 9/20
x = 8016/20
x = 400.8

Now, to calculate the expression:

3 - 456 + 56 : x + 2^3
-453 + 56 : 400.8 + 8
-397 : 408.8
-0.971 definitely subset (or -17/18)