Answer:
0.
or 2.12
Explanation:
To solve this, you have to split 28 into two of its factors. We can choose 7 and 4 (can be multiplied together to get 28):
28
7 4
However, 4 can also be split/factored:
28
7 4
2 2
So now we have a a pair of identical terms (being 2), so take this outside of the radical, like so (leave the 7 there):

0.
or 2.12