How to find value of λ easily & if the vectors are orthogonal good example (PART-3)

In this video explaining scalar product vector. In this example require value of λ and prove that vectors are orthogonal. This is very simple example.
When two vectors are orthogonal it means that they are perpendicular to each other. In other words their dot product is zero. Geometrically this means that the angle between the two vectors is 90 degrees (or pi/2 radians).

[HarshitKumar-flko]

Sir pls tell me why we didn't use cross product! Sir pls pls 🙏reply