Question 1135383

There are two keys to combining radicals by addition (or subtraction): look at the index, and look at the radicand. If these are the same, then addition and subtraction are possible. If not, then you cannot combine the two radicals.
Combining like terms:
as long as radicals have the same radicand (expression under the radical sign) and index (root), they can be combined. 
Sometimes you may need to add and simplify the radical. If the radicals are different, try simplifying first, you may end up being able to combine the radicals at the end.

compare the addition of
{{{7y+9y}}} ..........and.......... {{{7sqrt(y)+9sqrt(y)}}}-> in first expression variable {{{y}}} is in both terms, so just add constants {{{7+9 }}} 
same with second expression, the two radicals are the same , so just add constants {{{ 7+9 }}}