Answer:
BD - AC = 0 
Explanation:
Given that: 
BC = 36 , CD = 15 and BD = x 
As these are forming right angled triangle, 
Using Pythagorean theorem,

Taking square root on both sides 

As the given shape is rectangular, the sides parallel to each other will have same values.
AB = 15 , AD = 36 and AC = x 

Now,
BD - AC = 39 - 39 = 0 
Hence,
BD - AC = 0