Now suppose we equate this,
x=-3 ie. x+3=0
x=4 ie. x-4=0

Take some other random variable,and let this be 'y'.
Then we get

Here,x cannot be -3,since that would make the denominator 0,and division by zero is not allowed.
Now,what if 4 was put in this place? That would make the denominator=4+3=7.

So to solve this problem lets put y=7

If value of x in this is '-3',then the expression becomes void (division by 0).
Otherwise it becomes 7/7=0 which simplifies to 1=0,which is not possible and hence cannot be allowed.

In fact,any value put for y here gets equated to zero,so this expression anyways makes any value put for x null.

Hope this helps,
Prabhat