Answer
a. uss
b. dss
Work Step by Step
a. The $\Xi^o$ baryon has a strangeness of -2, so it contains two strange quarks. It is a neutral particle. Therefore the third quark is an up quark. $$\Xi^o=uss$$
b. The $\Xi^-$ baryon has a strangeness of -2, so it contains two strange quarks. It has a charge of -1. Therefore the third quark is a down quark. $$\Xi^-=dss$$
