Step 3 requires the principle that allows you to combine the measures of two angles that form a linear pair to sum up to 180 degrees. This principle is known as the "Angle Addition Postulate". According to the Angle Addition Postulate, if a point is in the interior of an angle, then the measure of the angle is the sum of the measures of the two angles formed.
Here is your detailed step-by-step solution:
1. Statements: [tex]\( m \angle TRV = 60^{\circ} \)[/tex]; [tex]\( m \angle TRS = (4x)^{\circ} \)[/tex]
Reasons: Given.
2. Statements: [tex]\( \angle TRS \)[/tex] and [tex]\( \angle TRV \)[/tex] are a linear pair.
Reasons: Definition of linear pair.
3. Statements: [tex]\( m \angle TRS + m \angle TRV = 180^{\circ} \)[/tex]
Reasons: Angle Addition Postulate.
4. Statements: [tex]\( 60 + 4x = 180 \)[/tex]
Reasons: Substitution property of equality.
5. Statements: [tex]\( 4x = 120 \)[/tex]
Reasons: Subtraction property of equality.
6. Statements: [tex]\( x = 30 \)[/tex]
Reasons: Division property of equality.
Therefore, the missing reason in step 3 is the Angle Addition Postulate.