I guess I learned it many years ago at school, but I must have forgotten it. From a geometry puzzle I got to the solution

$\sqrt{6-4\sqrt{2}} + \sqrt{2}$

My calculator tells me that (within its precision) the result equals exactly 2, but I have no idea how to transform the calculation to symbolically get to that result.

(I can factor out one $\sqrt{2}$ from both terms, but that does not lead me anywhere, either)