Over which interval is the graph of f(x) = –x2 + 3x + 8 increasing?

From the figure, the vertices of the given triangle are
(-3,4), (1,3) and (5,5).

The given transformation is (x,y) -> (x+3, y-2).
Therefore the coordinates of the transformed triangle are
(-3+3,4-2), (1+3,3-2), (5+3,5-2) = (0,2), (4,1), (8,3)

The transformed triangle is shown in red color.