First you need to know that the area of a square is side times side. So we know side times side is 8,100. Since both sides are the same length, solve by taking square root of 8,100, which is 90.
Now the distance from home plate to second base is the length of the diagonal of a square. The formula for that is square root of 2 time the side. Multiply 90 by square root of 2 and you get approximately 127.3 feet.