Please help with all steps

Dear Student
Angular momentum (L) = M V R

where R is the perpendicular distance between velocity and reference point .

Here since the line AB is straight, there would only 1 perpendicular distance from  reference point  i.e., OA

so, Angular momentum at A = M V R  

Angular momentum at  P = M V R 

Angular momentum at B = M V R

since, perpendicular distance between the line AB and O will always be R

so all the point lying on the line AB will have same angular momentum equal to M V R that is remains constant

Regards