Answer: 120.
Step-by-step explanation: The Pythagorean Theorem staes that a^2 + b^2 = c^2. Where a and b are side lengths and c is the length of the hypotenuse. We can thus say that 35^2 + b^2 = 125^2. This is simplified to 1225 + b^2 = 15625. We can simplify further to get b^2 = 14400. We can square root both sides to get b = 120.