Explaination :
According to the theorum. Both the angles in the diagram are vertically opposite angles. So we would keep both of them equal. After keeping them equal we would solve it and get the value of x.
[tex] \implies \: \sf{5x \: - 129 \: = \: 2x - 21 }[/tex]
★ Transposing 2x from R.H.S. to L.H.S., (Sign would be changed to negative).
[tex]\implies \: \sf{5x \: - 2x - 129 \: = \: - 21 } [/tex]
[tex]\implies \: \sf{3x - 129 \: = \: - 21 } [/tex]
★ Now, transposing -129 from L.H.S. to R.H.S. it would be in positive.
[tex]\implies \: \sf{3x \: = \: 129 - 21 } [/tex]
★ On substracting 21 from 129 we gets,
[tex]\implies \: \sf{3x \: = \: 108} [/tex]
[tex] \implies \: \sf{x \: = \: \dfrac{108}{3}} \\ \\ \implies \: \sf{x \: = \: \cancel\dfrac{108}{3}} \\ \\ \implies \: \sf{x \: = \: \dfrac{36}{1}} \\ \\ \implies \: \red{\bf{x \: = \:36 }}[/tex]
