Hello, Aiden,
If the baseball diamond is in the shape of a square and we want to find the diagonal (home base to second), the diagonal forms the hypotenuse of a right triangle with both sides of 90 feet. Use the Pythagorean Theory to find the distance.
(90)2 + (90)2 = X2
X2 = 16,200
X = 90 2(1/2) [The square root of 2]
X = 127.3 feet
Bob