Answer:
Step V: Transitive property of Inequality
Step VI: Subtraction Property of Inequality
Step-by-step explanation:
In Step IV, the RHS of t=both the sides are equal.
So, they equated the LHS of both the sides.
This is the transitive property of equality which states that if a = b and c = b then a = c.
In this case, a = 
b = 180⁰
c = 
Consequently, 
In step VI,
is subtracted on both the sides. So, this is called as Subtraction Property of Equality.