Recall for adding and subtracting variables, you can only add and subtract like terms.
Exp:
5x -4y +11x -y =
5x+11x -4y-y
=16x-5y
For adding and subtracting radicals, you combine the coefficients, ONLY WHEN THE RADICAND AND INDEX OR THE SAME!
Exp:
![8\sqrt[3]{8}-9\sqrt[3]{8}+2\sqrt[3]{8}](http://s0.wp.com/latex.php?latex=8%5Csqrt%5B3%5D%7B8%7D-9%5Csqrt%5B3%5D%7B8%7D%2B2%5Csqrt%5B3%5D%7B8%7D&bg=ffffff&fg=000000&s=0 "8\sqrt[3]{8}-9\sqrt[3]{8}+2\sqrt[3]{8}")
=
![1\sqrt[3]{8}](http://s0.wp.com/latex.php?latex=1%5Csqrt%5B3%5D%7B8%7D&bg=ffffff&fg=000000&s=0 "1\sqrt[3]{8}")