Side |CB|:
Sin(60)= opposite side / hypotenuse
We know hyp=10
And opposite side =|CB|
=sin60= opp/10
So multiply both sides by 10
10sin60 = opp
|CB| = 10 (√3 ÷ 2) = 5 √3
Then we can use Pythagoras theorem to get the other side (or we can do the same thing as before using either cos60 or sin30 =|BA| /10)
a² + b² = c² where c is the hypotenuse
(5 √3)² + b² = 10²
(25)(3) + b² = 100
75+ b² = 100
-75 from both sides
b² = 25
So b = |BA| = √25 = 5