Answer
$$ Q=4.269 \ M V$$
Work Step by Step
$$p_{T h}=-p_{a l p h a}$$
and use
$$K_{\alpha}=p_{\alpha}^{2}=p_{\alpha}^{2} / 2 m_{\alpha}$$
$$K_{T h}=\left(m_{\alpha} / m_{T h}\right) K_{\alpha}$$
$$Q=K_{\alpha}+\frac{m_{\alpha}}{m_{T h}} K_{\alpha}=\left(1+\frac{m_{\alpha}}{m_{T h}}\right) K_{\alpha}$$
$$\Rightarrow \quad Q=\left(1+\frac{4.00 u}{234 u}\right)(4.196 M e V)$$
$$\Rightarrow \quad Q=4.269 M V$$
