x/x-4=0
This equation cannot be solved because division by zero (x-4=0) is undefined.
x-4 cannot be equal to zero as the denominator of a fraction cannot be zero. Therefore, the solution to this equation is undefined.

2b^-9=0
To solve this equation, first we need to bring b to the numerator by taking the reciprocal of b^-9:
2/b^9 = 0

Now, to solve for b:
2 = 0*b^9
2 = 0
Since the equation 2=0 is false, there is no solution for this equation.