Answer
$22^{\circ}$.
Work Step by Step
Apply equation 27–7.
$$\lambda ‘=\lambda + \frac{h}{m_e c}(1-cos \phi)$$
$$\Delta \lambda = \frac{h}{m_e c}(1-cos \phi)$$
$$\phi = cos^{-1} (1-\frac{m_e c \Delta \lambda }{h})$$
$$\phi = cos^{-1} (1-\frac{(9.11\times10^{-31}kg) (3.00\times10^8 m/s)(1.7\times10^{-13}m)}{6.626\times10^{-34}J\cdot s})$$
$$\approx 22^{\circ}$$
