if you draw out a rectangle, two opposite sides will be 10 feet and the other two sides we will label as x.
The perimeter of a rectangle is adding up all the sides, so if you add up all those sides you get
x + x + 10 + 10
we know the perimeter is 28 feet, so we know
x + x + 10 + 10 = 28
now we can solve for x
x + x + 10 + 10 = 28
2x + 20 = 28
2x = 8
x = 4
one of the other sides is 4 feet