DH has done that before, so he's checking the Metro's website for you. They have a trip planner that works well (as long as you put the right info in...he put in a West when he shouldn't have, the other week, and got to walk a bonus 14 blocks.
).
The stop puts you on the outskirts of the mall, across the street from an REI. The mall is *huge*, and the parking lot is enormous, so allow yourself a LOT of time. I mean, it's not Mall of America huge, but still, big.
DH says it's the 150 Metro bus. I would look at their website and check it out on your own...metro.kingcounty.gov . After all, what if DH just put a "W" in? Shh, don't tell him I said that. (b/c I already have)
He says it's the same bus number to get back, but of course always check with your bus driver as you're getting off.