For this case we can pose the problem as a rectangle triangle.
 We know the hypotenuse and one side of the triangle.
 Therefore, the other side is given by:
 10 ^ 2 = 6 ^ 2 + x ^ 2
 Clearing x we have:
 x ^ 2 = 10 ^ 2-6 ^ 2
 x = root (10 ^ 2-6 ^ 2)
 x = 8 feet
 Answer:
 The length in feet of the garden is:
 x = 8 feet