Hey!
Well a soccer field would be a rectangle and if cut across you will be presented with two right triangles. So since you are presented with right triangles you will need to know at least two sides to determine the third. The formula used for right triangles is...
a² + b² = c²
Now a and b are the "legs" of the triangle meaning the two sides that come together to make a 90° angle. The c in the equation would be the hypotenuse of the triangle, otherwise the longest side or in this case the diagonal part. Now to find that distance you will have to plug in and solve...
100² + 230² = c²
10000 + 52900 = c²
62900 = c²
about 250.8 meters
Hope this Helped!
~A