Since the rectangles are similar. The ratio of their sides will be the same.
So, we can write:
[tex]x:18 = 4:6 \\ \\
\frac{x}{18}= \frac{4}{6} \\ \\
x = \frac{4}{6} * 18 = 12[/tex]
Therefore, the left side of the bigger rectangle will be 12 units in length.
