Let's solve this problem. We know the equation of the height of the ball that is:
[tex]f(x)=-0.02x^{2}+6x[/tex]
Where x represents the horizontal distance in yards the ball has traveled in the air. We know that a distance is always positive, so we conclude that x must be greater or equal than 0, so:
[tex]x \geq 0[/tex]
The horizontal plane represents the zero of the function, given that there is no possibility for the ball to get negative values, then [tex]f(x)[/tex] is also positive. Finally, from the graph, the appropriate domain is:
[tex]\boxed{0 \leq x \leq 300}[/tex]