Respuesta :

This is Pythagorean theorem. 
We already know a and c. 
a = 4 and c = 10
4² + b² = 10² 
16 + b² = 100 
Subtact 16 from each side. 
b² = 84 
Square root of each side: 
b = ≈9.165 or √84

Hope this helps!