What is the missing reason in step 3?
\begin{tabular}{ll|ll}
\multicolumn{1}{c|}{\textbf{Statements}} & & \multicolumn{1}{c}{\textbf{Reasons}} \\
\hline
1. [tex]$m \angle TRV = 60^{\circ}; \, m \angle TRS = (4x)^{\circ}$[/tex] & 1. & Given \\
2. [tex]$\angle TRS$[/tex] and [tex]$\angle TRV$[/tex] are a linear pair & 2. & Definition of linear pair \\
3. [tex]$m \angle TRS + m \angle TRV = 180^{\circ}$[/tex] & 3. & \textbf{Linear pair postulate} \\
4. [tex]$60 + 4x = 180$[/tex] & 4. & Substitution property of equality \\
5. [tex]$4x = 120$[/tex] & 5. & Subtraction property of equality \\
6. [tex]$x = 30$[/tex] & 6. & Division property of equality \\
\end{tabular}