List out the perfect squares: 1, 4, 9, 16, 25, .... We see that 21 is between 16 and 25. So the square root of 21 is between 4 and 5 because 4^2 = 16 and 5^2 = 25
Answer: between 4 and 5
On a calculator, you can compute sqrt(21) to get 4.58 approximately which is a value between 4 and 5