If a function is increasing over an interval, its derivative will be positive, and if it is decreasing it will be negative. I assume you know how to do differentiation, since you posted in calculus.

Of course, you can also use the equation of the parabola to determine intervals on which it is increasing or decreasing. In standard form, a parabola can be represented by the equation

where

is the vertex and

is the directed distance between the vertex and the focus. If

is positive, the parabola opens up, and if negative, the parabola opens down.

Since your parabola,

has a vertex at

and opens up, you can conclude that it is decreasing for

and increasing for

.