3a+4b - (-6a - 3b)?

Answer 1

Distribute the negative to both terms in the parentheses:
3a + 4b + (- -6a - - 3b)
Two negatives make a positive, so
3a + 4b + 6a + 3b
Combine like terms:
9a + 7b

Answer 2

The answer above is correct, of course, but I reckon you need to know how to do it. See if this works: If you're subtracting a parenthesis, just change the signs within the parenthesis, and eliminate the parenthesis.

Then you can add as you normally would. In this case, you get 3a + 4b +6a +3b. Does this work for you?

Ask yourself, what is minus minus six, and what is minus minus 3.