Note that this is a quadratic function whose graph opens down. You could plot this function and note the coordinates of the vertex; the y-coordinate would be the max value of the function.
Alternatively, find x = -b/(2a), where a = -2, b = 8 and c = -5. Then find the corresponding y-value.
Alternatively, find the derivative of f(x) and set it = to 0. Solve for x. Find the y value associated with this x. That y-value is the max of f(x).