Does this help? If it doesn't i can try to explain more if you want.
Perimeter= X + Y + W. Perimeter is the sum of all sides
Let x be the number you want to find
435= x times 25 times 3
435= 75x
X= 435/75 =5.8 (ans)
Then the number must be between 18² = 324 and 19² = 361. But is that what you want to know?
I am not sure I am right but I think it depends on the size of the X.