Test 1 Section 3 - #16 (page 400)

They're asking us to solve for an expression here (6a + 6b, to be precise) so we need to find a way to get from what they gave us to what they ask for.  Here's what we're given:
3a + 4b = b
We're already close.  Multiplying everything by 2 will get us closer (remember, use what they want as a GUIDE...that's what we're aiming for!):
2(3a + 4b) = 2b
6a +8b = 2b
Now, just subtract 2b from both sides, and we're there!
6a + 6b = 0
That's choice (A).

It's important to note that in this particular question, it's impossible to solve for the individual variables a and b. We'd need two equations to do that, and they only gave us one. Doesn't matter, though, since we aren't looking for the individual variables, and don't need them.  If I asked you for a cake, I'd want you to bring me a cake, not the ingredients to make one. Same goes for the SAT: if they ask for an expression, solve directly for the expression, don't try to find the ingredients.