Uniform electric field of magnitude $100V/m$ in space is directed along the line $y=3+x.$ Find the potential difference between points $A(3,1)$ and $B(1,3)$

Slope of line $AB,$

$m_1=\frac{y_2-y_1}{x_2-x_1}=\frac{3-1}{1-3}=-1$ and slope of electric field line is, 

$m_2=+1$

Since $m_1{m}_2=-1$

Therefore line $AB$ is perpendicular to electric field line. Hence $AB$ is an equipotential line.

$\therefore$ Potential difference = 0