I got this question on a coding interview.

Hanna moves in a lattice where every point can be represented by a pair of integers. She moves from point A to point B and then takes a turn 90 degrees right and starts moving till she reaches the first point on the lattice. Find what’s the point she would reach? In essence the problem boils down to finding the first point where the perpendicular to a line will intersect. How do we solve this?