The horizontal asymptote of a rational function tells us the limiting value of that function as it approaches infinity.
For a rational function to have a horizontal asymptote of

then,

The second condition is that,

Example are given in the graph above.
Here are some other examples,

3*1 = 3
3*2 = 9
3*3 = 27
3*4 = 81
3*5 = 243
3*6 = 729
The value of a would be 4.2, because the negatives cancel out each other.
The answer to your question is 3