1981 AHSME Problems/Problem 10

If $(p, q)$ is a point on line $L$ , then by symmetry $(q, p)$ must be a point on $K$ . Therefore, the points on $K$ satisfy $x=ay+b$ .Solving for $y$ yields $y = \dfrac xa-\dfrac ba$ . $\Longrightarrow \boxed{E}$