They are -4.2 and 4.2
You seem to be unaware of the fact that you could have obtained the answer much more easily and quickly by using the calculator that comes as part of your computer.
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This is simply done by entering 21 into the calculator, and hitting the square root sign. This answer, rounded to the nearest tenth is 4.6.
15.0
The square root of 18, rounded to the nearest integer, is 4.
The square root of 2 is an irrational number and rounded to the nearest tenth it is 1.4
The answer is 7.1