The question is asking you to find the values of "t" that make this true.
.. 13 = 5 +23t -16t^2
This is a quadratic equation that can be solved in any of the usual ways.
Usually, it works well to start by putting the equation into standard form.
.. -16t^2 +23t -8 = 0 . . . . . . . . subtract 13 from the above equation
t = (-23 ±√(23^2 -4(-16)(-8)))/(2(-16))
.. = (-23 ±√17)/-32
.. = 23/32 ±(√17)/32
.. = {0.59, 0.85}
The ball will be at a height of 13 feet when t=0.59 and t=0.85.
