since you were able to type that negative, I will assume that
f(x)=-x^2+3x+8
the vertex is where the graph changes from increasing to decreasing or vice verca
find theh vertex
-b/2a is x coordinate of vertex where
f(x)=ax^2+bx+c
a=-1
b=3
-3/2(-1)=-3/-2=3/2
we see that the leading coeficient (a) is negative so the graph opens down
it increases then decreases at the vertex
so from negative infinity to 3/2 is interval
in interval nottaion
(⁻∞,3/2]