A simple chalkboard sum has tripped up thousands - and the right answer depends on remembering one classroom rule you probably haven't used since middle school.

Learn how to form, solve, and rearrange equations using the tools and conventions of algebra with BBC Bitesize GCSE Maths.

In a boon to algebra students everywhere, a professor at Carnegie Mellon University has devised a simpler and more efficient way to solve problems involving the quadratic equation.