This question requires a knowledge of the meaning of the equilibrium equation and the meaning of the concentration terms in that equation.

The equilibrium equation is defined as K = product concentrations divided by the reactant concentrations...K = [products]/[reactants]...so you are correct in the way that you have set up the reactants and the products. Your error came about with respect to what should be done with the mole coefficients in the balanced chemical equation.  It makes quite a difference if you use these coefficients as simple multipliers rather than as exponents. Let's see what this mathematical difference is using some real numbers.

Suppose the concentration of the Ca(OH)₂ happened to be 2 moles per liter. If you use the "three" as a multiplier, the expression 3 x 2 moles/liter = 6 moles/liter. If you correctly use the "three" as an exponent [Ca(OH)₂]³, the expression [2 moles/liter]³ = 2x2x2 = 8 moles/liter. Imagine the magnitude of the error if you were dealing with water which has an exponent of 6...[2 moles/liter]⁶ = 2x2x2x2x2x2 = 64...whew, that's a huge difference.