I have simple equation, $a = b - c$. For instance $8 = 12 - 4$. If I want to compute $b$, I have to transform this equation to $b = a + c$. So we have three variable, $a=8,b=12,c=-4$. I calculated it this way: $12 = 8 + (-4) = 8 - 4 = 4$. Why the result is incorrect?