An electron beam is accelerated by a potential difference V to hit...

$K . E . = eV$,

$KE =$The kinetic energy of electron

The maximum energy of photon$= K . E .$

eV = \dfrac{hC}{{\lambda}_\min}

$⇒ {\lambda}_\minV = \dfrac{hC}{e} = constant$

Taken $log$ in both sides

\ln \left({\lambda}_\min\right) + \ln \left(V\right) = \ln \left(constant\right)

⇒ \ln \left({\lambda}_\min\right) = - \ln \left(V\right) + \ln ( constant )

$y = - ax + c$ the straight line of negative slope.