So we are required to make a rational expression.
Lets make this an expression in 'x'.
Then as per the question,-3 and 4 are not allowable replacements.
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
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