2##sqrt(21)##

Since 84 is not a perfect square, break it up into 2 factors. One of the factors should be the greatest perfect square integer that 84 is divisible by. In this case, that integer is 4. The other factor is 21, because 4 times 21 equals 84. At this point, the problem should look like ##sqrt4*sqrt21## . Now take the square root of 4, and you’ll get the correct simplified answer: 2##sqrt(21)##. (21 is not divisible by any perfect square integer, so you can’t further simplify the expression.) If you need the calculator answer, it’s approximately 9.16515138991