The type of a quadratic functions' vertex is determined by the coefficient in front of the -term. For the function it is positive. This means that the curve looks like a happy mouth, and, thus, it's an absolute minimum.
Again, look at the coefficient in front of Here it is , so negative, which means that the curve has the shape of an sad mouth, which means it has an absolute maximum.
In this function, there is no -term. This is because the function is linear. Linear functions does not have any vertex.
In there is an invisible in front of the -term. The term can then be written as Since is a positive number, the curve looks like a happy mouth, and then has a minimum point.