To simplify the radical, factorize the number under it using the prime number
Since 252 is an even number, then let us divide it by the 1st prime number 2
since 126 is an even number, then divide it by 2 again
Now 63 is an odd number and sum of its digit = 6 + 3 = 9, then
Divide it by the 2nd prime number 3
Since 21 is divisible by 3, divide it by 3 again
Then 252 = 2 x 2 x 3 x 3 x 7, put them under the radical
Each number repeated twice can go out the radical, then
2 and 3 will go out the radical
The simplest form of the radical is 6 square root 7