Answer
The aperture width is $~~8.0~cm$
Work Step by Step
We can use similar triangles to find the aperture width $w$:
$\frac{w}{2.00~m} = \frac{12.0~cm}{3.00~m}$
$w = \frac{(2.00~m)(12.0~cm)}{3.00~m}$
$w = 8.0~cm$
The aperture width is $~~8.0~cm$
