You need to add a -1
We can find this by setting up the equation like so
x + (-6) + 12 + 15 / 4 = 5
x + 21 = 20
x = 20 -21
x = -1
Now if we insert -1 where x is located in the equation, you will get a means of 5
The digit one is in the "ten thousands" column
You got the b part wrong. It would be 4B+4C/2=4D
