An electron of mass m and magnitude of charge $|e|$ initially at rest gets accelerated by a constant electric field E.

(4) Acceleration of electron in electric field, $a=\frac{e E}{m}$

Using equation

$v=u+a t$

$\Rightarrow v=0+\frac{e E}{m} t$

$\Rightarrow v=\frac{e E t}{m}$      ...(i)

De-broglie wavelength $\lambda$ is given by

$\lambda=\frac{h}{m v}=\frac{h}{m\left(\frac{e E t}{m}\right)}$         [using (i)]

$\Rightarrow \lambda=\frac{h}{e E t}$

Differentiating w.r.t. $t$

$\frac{d \lambda}{d t}=\frac{d\left(\frac{h}{e E t}\right)}{d t}$          $\Rightarrow \frac{d \lambda}{d t}=\frac{-h}{e E t^{2}}$