a + b + c = 180 for b We have a literal equation. Subtract (a + c) from each side of the equation to isolate b: a + b + c - (a + c) = 180 - (a + c) The (a + c) cancels on the left side, so we have: b = 180 - (a + c) or, distributing the negative sign: b = 180 - a - c