Explanation:
A baseball field is in the shape of a square (or diamond). Home plate and second base are opposite corners of each other. So the distance is equal to the diagonal of the square.
Use Pythagorean theorem to find the diagonal, or use properties of a 45-45-90 triangle.
Using Pythagorean theorem:
c² = a² + b²
c² = (90 ft)² + (90 ft)²
c = 90√2 ft
c ≈ 127 ft