Remember that A squared plus B squared equals C squared.
Since sides F_D and D_E are both 3, the answer cannot be 4.2 or 2.4.
SO basically you just add 3 + 3 = (The side F_E, which you can get by putting numbers in the formula), you should get your answer.
3² + 3² = Line EF²
9 + 9 = Line EF²
18 = Line EF²
√18
You get the idea. Hope this helps!