Question

Given: mTRV = 60° mTRS = (4x)° Prove: x = 30
What is the missing reason in step 3?
substitution property of equality
angle addition postulate
subtraction property of equality
addition property of equality

Answer 1

the answer is angle addition postulate 
proof
mTRV   +  mTRS = 180°, both angles are called supplementary angles, their sum is 180°

Answer 2

The Angle addition Postulate   :The postulate describes that putting two angles side by side with their vertices together creates a new angle whose measure equals the sum of the measures of the two original angles.Given the sum of a pair of angles and the algebraic expressions that represent them we can form and solve an equation.

It is given mTRV = 60° mTRS = (4x)° Prove: x = 30'

In statement 3 we have m<TRS+M<TRV=180(The angles add to given 180)

The missing reason in step 3 is angle addition postulate.