Math in Reideen

Here is a screenshot from the 4 minute mark in Reideen, a remake of the original Reideen anime from the 70's. Besides the fact that student multiplied by a binomial including a radical ONLY to resort to the quadratic formula, can you spot the error?

Also, I do not condone doing that much mental math when multiplying polynomials involving radicals. That's how you make errors. (The final answer is correct, but one of the equations is incorrect.)


I'm also somewhat disturbed by this "hint" from a friend. I hope it's a joke from the fansubbers...