Answer:
b = 60
Step-by-step explanation:
Using Pythagoras' identity in the right triangle
The square on the hypotenuse is equal to the sum of the squares on the other 2 sides, that is
b² + 63² = 87²
b² + 3969 = 7569 ( subtract 3969 from both sides )
b² = 3600 ( take the square root of both sides )
b = 
= 60
