you need to use this particular formula
a^2 + b^2 = c^2
this allows you to solve for the sides of a triangle which is what the diagonal would look like if u were to draw it. a equals the width of the park and b equals the height of the park.
6^2 + 8^2 = c^2
next you need to multiply 6 by the power of 2
36 + 8^2 = c^2
then the same for the 8
36 + 64 = c^2
then add the numbers together.
100 = c^2
after you add you get the square root of 100 which is 10. a square root is 2 numbers that multiply together to equal a particular number. square root is what you do to cancel out a power of 2
so if you walk diagonally you go 10 miles instead of 14 miles leaving a difference of a whole 4 miles.
hope I helped