The space diagonal will have length ...
... d = √(1² +4² +2²) = √(1 +16 +4) = √21
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This can be found using the Pythagorean theorem. A drawing can help. Find the length of any face diagonal, then use that length as the leg of a right triangle whose hypotenuse is the space diagonal and whose other leg is the edge length not used in the first calculation.