Hey there!!
Given equation :
A + B + C = D
and solve for B
In order to solve for the B, we will need to take all the other variables to the other side.
... A + B + C = D
Subtracting A on both sides.
... B + C = D - A
Subtracting C on both sides.
... B = D - A - C
Hence, we got the value for B.
Hope my answer helps!!