Bagaimana cara menyelesaikan nomor 7&10 ?.. Terimakasih

Vektor
7.
a (1, -2, 1)
b (2,-2, -3)
c (-1,1 , 2)

2a - 3b - 5c = 2 (1, -2, 1) - 3(2, -2, -3) - 5(-1, 1 , 2)
2a - 3b - 5c = (2, - 4, 2) + (-6, 6 , 9) + (5,- 5, -10)
2a - 3b - 5c = (2-6+5)i +(-4+6-5)j +(2 +9 -10)k
2a - 3b - 5c = (1) i +(-3)j +(1) k
2a - 3b - 5c = i - 3j + k

10.
AB = b-a = (3,3,1) -(1,2,3) = (2,1,-2)--> |AB|= √(2²+1²+(-2)²= √9= 3
BC = c-b = (7,5,-3)-(3,3,1) = (4,2,-4)--> |BC| = √(4²+2²+(-4)² = √36 = 6
AB : BC = 3 : 6 = 1 : 2