You've provided the equilibrium concentrations of the products and reactants, but you haven't given the balanced chemical equation for the reaction. To calculate the equilibrium constant (K), we need the balanced equation.

Here's how to calculate K once you have the equation:

1. Write the Balanced Equation: The balanced equation will tell you the stoichiometric coefficients for each species involved in the reaction.

Example:

```

2 H₂O₂ (aq) ⇌ 2 H₂O (l) + O₂ (g)

```

2. Write the Equilibrium Constant Expression: The equilibrium constant expression is written using the concentrations of the products divided by the concentrations of the reactants, each raised to the power of their stoichiometric coefficient.

For the example above, the expression is:

```

K = [H₂O]² [O₂] / [H₂O₂]²

```

3. Plug in the Equilibrium Concentrations: Substitute the given equilibrium concentrations into the expression.

```

K = (0.21)² (0.25) / (0.15)²

```

4. Calculate K: Solve for K.

Important Note:

* States of Matter: For the equilibrium constant expression, only include the concentrations of species in the aqueous (aq) and gaseous (g) phases. Solids and liquids have constant concentrations and are not included in the expression.

* Units: The equilibrium constant is unitless.

Please provide the balanced chemical equation for the reaction, and I can help you calculate the equilibrium constant.