Question

two objects are thrown from the same point into the opposite direction (both directed horizontally) as shown on the picture with initial velocities V1=3 m/s and V2 = 4 m/s. find distance l between the objects at the moment when they have mutually perpendicular velocities.

Answer 1

is there any picture?

Answer 2

they will be at same horizontal level because thier centre of mass is freeling falling since there is no external force on the system of two object.
when both the velocities are perpendicular, their dot product is zero. use this to find the time. and then from there you can find the distance between the two objects.