two rectangles to be similar, their sides have to be proportional (form equal ratios). The ratio of the length should equal the ratio of the width.
thus,
[tex]\begin{gathered} \frac{12}{x}=\frac{6}{10} \\ 12\cdot10=6x \\ 120=6x \\ x=\frac{120}{6} \\ x=20 \end{gathered}[/tex]
Therefore x=20.
