Hi pepo,
In the Quant section, you're going to end up using Algebra and Arithmetic skills on most questions, so making sure that you know all of those math rules (and can do the math correctly by hand) is a must. Data Sufficiency is another big category (you'll likely see about 15 DS questions on Test Day) and those questions revolve around skills that are not, strictly-speaking, 'math skills' - organization, accuracy, attention-to-detail, thoroughness, the ability to prove that your idea is correct, etc.
With a Q39, you're missing out on points in a number of different areas AND you're likely making silly/little mistakes along the way. As such, sequences should NOT be on your mind right now. There are too many points to be picked up in other areas.
How long have you been studying?
What materials have you been using?
GMAT assassins aren't born, they're made,
Rich