You are looking for the x-value of the vertex.
An easy way to find this point is to graph it.
If you do not want to graph it, then memorize the formula for finding the x-value of the vertex: x = -b/(2a).
Because your expression is written correctly, a = -25, b = 75, and c = 24.
Therefore, the x-value of the vertex is -(75) / (2·-25) = -75 / -50 = 1.5 <--- the time.
To find the height, replace x wby 1.5 in the original expression.