Abcd is a rectangle. find the length of each diagonal.
ac=3y/5
bd=3y-4

The diagonals of a rectangle are equal, so

, then multiply both sides by 5
3y = 5(3y - 4) Distribute the 5
3y = 15y - 20 Subtract 15y from both sides
-12y = -20 Divide both sides by -12
y = 

So, AC = 
and BD = 3(5/3) - 4 = 1