The given graph is parabola graph with vertex at the point (2.0,24) and branches of the parabola go in a negative direction of y-axis.
As known parabola is symmetric about her axis of symmetry and in this case the symmetry axis has equation t=2.0 (vertical line). Thus, the height of the javelin above the ground is symmetric about the line t=2.0.
From the graph you can see that the first time when javelin is 20 feet above the ground is when t=1.0 and the second time is t=3.0 (to determine this you should draw horizontal line through point (0,20) and consider two points of intersection (1.0,20), (3.0,20) between parabola and horizontal line).