Answer
$$K = \frac{a_{Cu^{2+}}a_{H_2}}{a{^2}_{H^+}}$$
$$K \approx \frac{[Cu^{2+}]_{eq}P_{H_2}}{[H^+]_{eq}^2}$$
Work Step by Step
$$K = \frac{a_{Cu^{2+}}a_{H_2}}{a{^2}_{H^+}}$$
$$a_{Cu^{2+}} = \frac{\gamma_{Cu^{2+}}[Cu^{2+}]_{eq}}{c^o}$$
$$a_{H_2} = \frac{\gamma_{H_2}P_{H_2}}{P^o}$$
$$a_{H^{+}} = \frac{\gamma_{H^{+}}[H^{+}]_{eq}}{c^o}$$
$$K = \frac{(\frac{\gamma_{Cu^{2+}}[Cu^{2+}]_{eq}}{c^o})(\frac{\gamma_{H_2}P_{H_2}}{P^o})}{(\frac{\gamma_{H^{+}}[H^{+}]_{eq}}{c^o})^2} \approx \frac{[Cu^{2+}]_{eq}P_{H_2}}{[H^+]_{eq}^2}$$
