Question

the length of a rectangle is 3 times its width and the length of its diagonal is 6√10 cm the perimeter of the rectangle is

Answer 1

Let the width be x. Then length is 3x. 
Notice that when we take two sides with its diagonal we have a right triangle. 
Apply pythagorean theorem. 
 
 
 
 
 <---width 
Then our length is 18. 

P = 2L + 2W 
P = 2(6) + 2(18) =  cm