help! give solutions and answer in english please :((

Math Senior High School
→→ Vektor ←←

Pembahasan :
Perpendicular
(a - b) . (6a + b) = 0
6|a|² - 5ab - |b|² = 0
6(2)² - 5ab - 3² = 0
6(4) - 9 = 5ab
5ab = 15
ab = 3

So...
Cos β = (ab)/(|a| |b|)
Cos β = 3/(2 . 3)
Cos β = 3/6
Cos β = 1/2
Cos β = Cos 60°
β = 60°