Answer
If we double the speed, the de Broglie wavelength decreases by a factor of $\frac{1}{2}$
Work Step by Step
We can use Equation (38-20) to find an expression for the de Broglie wavelength:
$\frac{1}{2}mv^2 = \frac{h^2}{2m \lambda^2}$
$\lambda^2 = \frac{h^2}{m^2~v^2}$
$\lambda = \frac{h}{m~v}$
If we double the speed, the de Broglie wavelength decreases by a factor of $\frac{1}{2}$
