I was always told to "divide by the highest power of x, if that fails, divide by the next highest"
I divided by the highest power of x and got like 1/0 which doesn't work.
I then divided by x:
\frac{\frac{9}{x}+\frac{x^{2}}{x}}{\frac{3x}{x}}
Cancelling down, then substituting x=infinity in...