Find the differential equation of all non-vertical lines in a plane.

We know that, the equation of all non-vertical lines are y = mx + c

On differentiating w.r.t. x, we get

dy/dx = m

Again, on differentiating w.r.t. x, we have

d2y/dx2 = 0

Thus, the required equation is d2y/dx2 = 0.