Question

Find the interval(s) where the function is increasing and the interval(s) where it is decreasing. (Enter your answers using interval notation. If the answer cannot be expressed as an interval, enter EMPTY or ∅.)
f(x) = 5x2 + 9x + 8

Answer 1

This is just a parabola

When a function increases, its derivative is positive.
When a function decreases , that derivative is negative

if y = 5x^2 + 9x + 8
dy/dx = 10x + 9

When 10x+9 > 0
x > -9/10

when 10x + 9 < 0
x < -9/10

You do the interval notation stuff, I am "old school" and I use
the above notation.

( bet you the x of the vertex is -9/10)