-8a^2+5ab-12a^2-6(2a^2-4ab-10n^2)
Is the n^2 supposed to be an a^2 ?
If so, you can combine three pairs of terms. Otherwise, two pairs.
Just multiply each term in parentheses by 6 and combine "like" terms.
You may also able to factor, after that.
You should try this yourself instead of looking for a web site (including jiskha) to do it for you.