Bagaimana cara menyelesaikan nomor 19 & 21 ?.. Terimakasih

19) u(v + u) = u.v + u.u
= |u| |v| cos 60° + |u| . |u|
= 4 . 2 . 1/2 + 4 . 4
= 4 + 16
= 20

20) a tegak lurus b maka
a.b = 0
1(x) + 2(5) + 3(6) = 0
x + 10 + 18 = 0
x = -28

21) c = a - b = (7, 5, -3) - (5, 2, 3) = (2, 3, -6)
|c| = √(2^2 + 3^2 + (-6)^2) = √(4 + 9 + 36) = √49 = 7
vektor satuan c = 1/|c| . c
= 1/7 (2, 3, -6)
= (2/7, 3/7, -6/7)
= 2/7 i + 3/7 j - 6/7 k