[tex]\qquad\qquad\huge\underline{{\sf Answer}}[/tex]
According to properties of arc on a circle, angles subtended by an arc on circle at its boundaries are equal to one another.
Hence ~
[tex] \qquad \dashrightarrow \tt \: \angle C = \angle W [/tex]
[tex]\qquad \tt \dashrightarrow \:12x + 15 = 3x + 78[/tex]
[tex]\qquad \tt \dashrightarrow \:12x - 3x = 78 - 15[/tex]
[tex]\qquad \tt \dashrightarrow \:9x = 63[/tex]
[tex]\qquad \tt \dashrightarrow \:x = 63 \div 9[/tex]
[tex]\qquad \tt \dashrightarrow \:x = 7[/tex]
Therefore, value of x is 7
