ANSWER
[tex]x=11[/tex]
EXPLANATION
The figure given is a square.
Each angle in a square is 90 degrees and the diagonals bisect each angle.
This means that :
[tex]\begin{gathered} 6x-21=45 \\ \text{Collect like terms:} \\ 6x=45+21 \\ 6x=66 \\ \text{Divide through by 6:} \\ x=\frac{66}{6} \\ x=11 \end{gathered}[/tex]
That is the value of x.
