A vertical asymptote simply means that the function is undefined at a certain point. We are given that it occurs at x=25. Notice that x is in the denominator and if the denominator equals 0, the entire function is undefined. a and k are simply constants, and so they can take on any value. This is one of infinite solutions to this question:
Notice that when you plug in x=25, the denominator is 0 and the function is undefined making it a vertical asymptote.
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