For (iii), we can use the first derivative test or the second derivative test.

First derivative test simply tests for the signage (positive or negative) of the areas around the turning point, as per what I wrote.

Second derivative test is an alternative way to obtain the nature of the stationary point. The function dy/dx is differentiated one more time, and then the value of x of the particular stationary point is substituted into the expression for d2y/dx2.

Depending on the complexity of the function to be differentiated, one method may be easier than the other method. In general, the more difficult it is to differentiate a function, the more likely we should use the first derivative test.

Note that if the second derivative test yields a value of 0 at the stationary point, the result is 'inconclusive' and the second derivative test is said to have 'failed' or is 'unsuccessful', in which case we would then have no choice but to rely on the first derivative test.