Mistakes: The student did not distribute the 3 into 2, and the student did not distribute the 6 into negative 2. The student also left out the negative 2x in the first half of the equation.
You could work this out with the pythagorean theorem, by proving that 6^2, 36, plus 8^2, 64, equals 100. The fastest way, however, is to use pythagorean triples. These are predetermined sets of numbers that work as side lengths for right triangles. The first two are 3, 4, and 5, which form a right triangle, and 6, 8, and 10, shown here.