To find the value of the missing angle, use Pythagoras theorem
a² = b² + c²
Where a is the hypotenuse, b opposite side and c the adjacent side
So in this case we need to identify the sides
The opposite side is 9.9 and the adjacent side is 5
Then replace on the formula
a² = b² + c²
a² = (9.9)² + (5)²
a = /