Answer:
by rule SAS
=======================================================
Step-by-step explanation:
The tickmarks tell us that segments OS and TS are the same length. This represents one "S" in "SAS". The other "S" refers to segment AS being congruent to itself. We can say AS = AS by the reflexive property.
The pair of congruent angles is angle AST and angle ASO. This is shown by the arc markings. Note how the angles are between the congruent sides mentioned in the previous paragraph. So this is why the SAS congruence rule applies.
The order of SAO is important. We have S listed first because it's listed first in SAT. The next letter is A for similar reasons. Instead of listing T, we go for O instead.