1. In fact, the projection of B on A is the projection length on A perpendicular to the endpoint of B vector, and the projection length = | b | cos.

= | b | * ab/(| a | | b |)= ab/| a | = 12/3 = 4

2. Projection length = acos

In fact, the projection length is negative, because it may be on its extension line, or it may be negative from the perspective of cos.

I hope I can help you, thank you!