To find the constant of proportionality, we need to determine the ratio of \(y\) (inches of shelf space) to \(x\) (number of cereal boxes). This ratio should be the same for each pair of values.
Using the given data points:
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For \(x = 2\) and \(y = 6\): \[ \text{Ratio} = \frac{y}{x} = \frac{6}{2} = 3 \]
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For \(x = 4\) and \(y = 12\): \[ \text{Ratio} = \frac{y}{x} = \frac{12}{4} = 3 \]
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For \(x = 5\) and \(y = 15\): \[ \text{Ratio} = \frac{y}{x} = \frac{15}{5} = 3 \]
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For \(x = 6\) and \(y = 18\): \[ \text{Ratio} = \frac{y}{x} = \frac{18}{6} = 3 \]
In every case, the ratio \(\frac{y}{x} = 3\).
Thus, the constant of proportionality is \(3\).