#### Answer

Maximum
$y=-1$

#### Work Step by Step

The function will face downwards, since the $a$-value is negative, meaning that the function will have a maximum and then continue down from that point to infinity.
As for the value of that maximum, without the vertical shift $1$ unit down, the maximum of the function would be $(0,0)$. However, there is a vertical shift that moves the function down one unit. Therefore, the maximum value of the function is $y=-1$.