you wrote that you had gotten to this point: x/(.15) = 1.8x10^-5
I think you need to take a look at the numerator "x" -- you've forgotten something there. take a look at sodium acetate product and what's left after you do the "5% approximation".
Also, that's the Ka of acetic acid. Did you mean to write out HOAc in your equation? Just wonderin...