In Exercises $59-66,$ find the critical points, domain endpoints, and
extreme values (absolute and local) for each function.
$$
y=\left\{\begin{array}{ll}{-\frac{1}{4} x^{2}-\frac{1}{2} x+\frac{15}{4},} & {x \leq 1} \\ {x^{3}-6 x^{2}+8 x,} & {x>1}\end{array}\right.
$$