This function is a line, so it has no critical points.
Better yet, we don't even need to evaluate f at the endpoints. Since f is a line with a slope of 3, f is an increasing function. This means f must be smallest at the start of the interval and largest at the end of the interval.
The global min of f on this interval occurs at x = -5 and the global max of f on this interval occurs at x = 5.
These problems become more complicated when we start doing something called optimization, which is the same exact math but with more words added on top.