X=3 is a vertical line . The question is "Is this line a vertical asymptotefor the function f(x)?". Vertical asymptote is vertical line near which the function grows without bound. The function f(x)=5x-15/x-3 is not asymptotic to the line x=3, because for x=3 f(x)=5*3-15/3-3=0/0=0 . So, there is a value for x=3 and the function f(x) does not grow near x=3 without bound.
