Angular momentum L is given by r X P where r is the perpendicular distance and P is the momentum. Hence our r in the equation is the distance a. So we have r=a= (7, 5, 0)/r . I had to divide by "r" because the vector appears as ra and not r, and. P = (-10, 15, 0) Taking the cross product.
i (5·0 - 0·15) - j (7·0 - 0·(-10)) + k (7·15 - 5·(-10))/r = = i (0 - 0) - j (0 + 0) + k (105 + 50)/r= {0; 0; 155/r}