__Example 1__Find the maximum or minimum value of each of the following quadratic function by completing the squares. In each case, state the value of x at which the function is maximum or minimum. And also, state the maximum or minimum point and axis of symmetry for each case.

(a) $f(x)={x}^{2}+6x+7$

(b) $f(x)=2{x}^{2}-6x+7$

(c) $f(x)=5-2x-{x}^{2}$

(d) $f(x)=4+12x-3{x}^{2}$