Find the nonpermissible replacement for y in this expression y+7/y-3

In an expression like this, the denominator (bottom) of the fraction cannot have the value 0.  Division BY zero is undefined. The question becomes, then, what value of y makes y - 3 = 0?

Solve that equation for y by adding 3 to both sides, and you get y = 3. That is the nonpermissible replacement for y.