The problem is asking you to find tha height of the ball after 0.75 seconds, and they give you the formula for the height:
[tex]h(t)=-16t^2+10\text{ t + 6}[/tex]So since this a function in terms of the time "t", all you need to do is to replace t with "0.75" in the equation, and thus obtain the height of the object at that moment in time:
[tex]h(0.75)=-16(0.75)^2\text{+10 (0.75)+6=}4.5[/tex]The answer comes in feet, and it is: 4.5 ft
