Find analytically the exact critical point of the function

Question

Question: Find analytically the exact critical point of the function which represents the time, t, to walk to the bus stop in Example. Recall that t is given by

t = (x/6) + √((2000 - x)² + 600²)/4

Example: Find the range of the function f, given the domain from Example 1. In other words, find all possible values of the chirp rate, C, in the equation C = f(T).