Final answer:
The missing reason in step 3 is the Substitution property of equality. This property allows us to replace one quantity with another if they are equal.
Step-by-step explanation:
The missing reason in step 3 is Substitution property of equality. The Substitution property of equality states that if two quantities are equal, then one can be replaced with the other in any expression. In this case, the angle TRS is equal to 4x degrees, so we can substitute it with 60 degrees.