Coterminal angles are angles in standard position (angles with the initial side on the positive x-axis) that have a common terminal side.
In another words, are angles "located at the same place". We can find coterminal angles by adding or subtracting 2π from our angle.
First, let's add 2π.

π/4 is a positive angle that is coterminal to -7π/4.
Now, let's subtract 2π.

-15π/4 is a negative angle that is coterminal to -7π/4.