Multiply -1 (the - sign is equal to -1) to both numbers in the parenthesis.
-1 * 3x = -3x
-1 * -8 = 8
So now the equation looks like....
10x - 3x + 8
Now combine like terms.
10x - 3x = 7x
7x + 8 is the answer.
**I had a hard time reading what you wrote there because it was not really that clear. Perhaps giving me a screen shot next time would make it easier for me. Thanks!