Question 877951
(-9+ sqrt(21))/6
{{{(-9+ sqrt(21))/6}}} cannot be simplified because the terms of the numerator cannot be combined without approximation, and because all three separate numbers have no factors in common.  Stated more plainly, -9 and sqrt(21) and 6 have no common factor. 


Do not confuse the square root with the input of the square root.  Seven is not a factor of {{{sqrt(21)}}}.