Solve for X:
X= B - A
A + X = B
Subtract A from both sides:
\qquad
\begin{eqnarray}
A + X &=& B \\ \\
A + X \red{- A} &=& B \red{- A}
\end{eqnarray}
X + A = B
Subtract A from both sides:
\qquad
\begin{eqnarray}
X + A &=& B \\ \\
X + A \red{- A} &=& B \red{- A}
\end{eqnarray}
B = A + X
Subtract A from both sides:
\qquad
\begin{eqnarray}
B &=& A + X \\ \\
B \red{- A} &=& A + X \red{- A}
\end{eqnarray}
B = X + A
Subtract A from both sides:
\qquad
\begin{eqnarray}
B &=& X + A \\ \\
B \red{- A} &=& X + A \red{- A}
\end{eqnarray}
Simplify.
\qquad X = B - A