(11m^2-2mn+8n^2)+(8m^2+4mn-2n^2)
ok. The original problem:
$\displaystyle 11m^2-2mn+8n^2+8m^2+4mn-2n^2$
now, you are to combine like terms. This means to combine the terms that have the same variables being multiplied to each other. Look at $\displaystyle 11m^2$ you will combine this with $\displaystyle 8m^2$ because they both have everything exactly the same, except for the coefficient(the number). so, you will get $\displaystyle 19m^2$, and you do this for the whole problem to get:
$\displaystyle 19m^2+2mn+6n^2$
YAY
Once again, you are adding and subtracting the numbers that have the same variable ending, simply put.
Many students find it helpful to add polynomials vertically:
[html]11m^2 - 2mn + 8n^2
8m^2 + 4mn - 2n^2
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19m^2 + 2mn + 6n^2[/html]