The square root of 324 is 18. Do you mean the square root? Or do you mean the root of 324, squared? If you do, and by "root" you mean "square root", then 324.

If you mean, the square root of 4 multiplied by the square root of 5, that is 2x5=10 If you mean, the square root of ( 4x25), that is the square root of 100 which is 10