Answer
$-4.5m$
Work Step by Step
We can find the focal length as
$f=(\frac{1}{d_{\circ}}-\frac{1}{d_i})^{-1}$
We plug in the known values to obtain:
$f=(\frac{1}{\infty}+\frac{1}{-(4.5m-0.021m)})^{-1}$
$f=-4.5m$
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