Answer
The exponent for the hydroxide ion concentration is wrong; it should be equal to 2.
Correct version: $K_{sp} = [Fe^{2+}][OH^-]^2$
Work Step by Step
Each $Fe(OH)_2$ has 2 $OH^-$ in it. Therefore, its $K_{sp}$ expression should have 2 as the exponent of the hydroxide concentration "$[OH^-]$" .
