When two vectors sum to zero how must they be related?

They must have equal magnitude and opposite direction.

In fact, the sum of two vectors can be calculated by decomposing the two vectors along the two directions (horizontal and vertical). If the sum of the two vectors is zero, it means that the x-component of the first vector is equal and opposite to the x-component of the second vector; similarly, the y-component of the first vector must be equal and opposite to the y-component of the second vector. This means that the two vectors must have equal magnitude and they have to be in opposite directions.