The number 42 has two square roots: 6.48 and -6.48, each accurate to two decimal places. The positive root, 6.48, is called the principal square root.
The square root of 42 is the number that can be multiplied by itself to equal 42. Most calculators have a function that calculates the square root of any positive number. One can estimate the square root of 42 by noting that 6 squared is 36 and 7 squared is 49. These results imply that the principal square root of 42 lies somewhere between 6 and 7. One can then narrow down the square root through a trial and error process.