How did the Indian subcontinent "drift"
into the Eurasian
continent? Which
forces moved them
together?

Indian subcontinent was drifted into Eurasian continent by the rotation of earth. Earlier, the drift or the shifts in tectonic plates were not believed 'cause, people assumed that the mass are too large to shift.

reflecting point a across the origin negates both of its coordinates.

b = (5, -4)