Answer
Please see the work below.
Work Step by Step
We know that
$c_m=\frac{\Delta T_f}{K_f}$
We plug in the known values to obtain:
$c_m=\frac{0.086C^{\circ}}{1.858\frac{C^{\circ}}{m}}=4.628\times 10^{-2}m$
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