To simplify a radical, first we find numbers that multiply by each other to get the given number (in this case, 28) while having one of the numbers as a number that has a square root.
We see that 7*4 equals 28, and 4 is a number that has a square root, so we'll do:
[tex] \sqrt{4*7} [/tex]
Which is [tex]2 \sqrt{7} [/tex] (because we found the square rot of 4 to be 2, therefore the two get moved outside the radical).
:)
