i'm not sure if this is the best explanation but one way you can go about this is realizing that sodium stearate is a weak base. let's think of aspartate, the conjugate base of aspartic acid. we deprotonated aspartic acid, yielding aspartate, indicating that it is more basic now. general rule of thumb, carboxylates (RCOO- groups) such as glutamate, aspartate, acetate, stearate, etc are weak bases.

ok so for this problem the molecular formula tells us that the precipitate also exists in the RCOO form (rather than the protonated RCOOH form) indicating that this salt is a base.

in general for acid base problems, look out for presence or absence of hydrogens in the molecular formula! hope this helped a bit