You need to know the location of the vertex, which is the highest point.
Your expression, -25t² + 75t + 24 has the form at² + bt + c, so a = -25, b = 75, and c = 24.
The x-vaue of the vertex can be found by using this little formula: x-value = -b/(2a).
For this problem, the x-value of the vertex is -75/(2·-25) = -75/-50 = 1.5
This represents the time of the maximum height; time = 1.5.