To estimate square roots you find 2 integers who's squares are closest/in between (one smaller and one larger but only have a difference of 1, ex. 4&5) the number, in this instance 120.
You know that 10^2 is 100 and
You know 11^2 is 121 so the square root is in between 10 and 11 but closer to 11 so your estimate is 11