To determine which of the given options represents a function that is not linear, we need to identify the equations and their forms.
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\(y = \frac{1}{6}x + \frac{2}{3}\): This is a linear function in the form \(y = mx + b\) where \(m = \frac{1}{6}\) and \(b = \frac{2}{3}\).
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\(9x + 3y - 18 = 0\): This equation can be rearranged to find \(y\): \[ 3y = -9x + 18 \ y = -3x + 6 \] This is also a linear function.
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\(y = x^2 - 3x + 2.25\): This equation is a quadratic function due to the \(x^2\) term. Hence, it is not linear.
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\(2x + 3y = 16\): This can be rearranged to solve for \(y\): \[ 3y = -2x + 16 \ y = -\frac{2}{3}x + \frac{16}{3} \] This is a linear function.
From the analysis, the only function that is not linear is:
\(y = x^2 - 3x + 2.25\).