Try this solution:
There are several ways to find the max or min of the given function:
1. to use derivative of the function. For more details see the attachment (3 basic steps); the coordinates of max-point are marked with green (-5; 14.5)
2. to use formulas. The given function is the standart function with common equation y=ax²+bx+c, it means the correspond formulas are (where a<0, the vertex of this function is its maximum):
Finally: point (-5;14.5) - maximum of the given function.
3. to draw a graph.
