Find the perimeter and area of a rectangle with a length of 6 feet and a diagonal of 10 feet

Use the Pythagorean Theorem to find the other side length, as the triangle formed by the 6ft and the 10ft side is a right triangle.
6^2 + ?^2 = 10^2
36 + ?^2 = 100
?^2 = 100 - 36
?^2 = 64
? = 8
Now you know the rectangle is 6ft by 8ft, you can find the area by multiplying the two, and the perimeter by doubling both then adding.
Final
area = 6 * 8 = 48ft^2
perimeter = 6*2 + 8*2 = 28ft 
Hope I helped :)