A meteoroid is speeding through the atmosphere, traveling east at 23.1 km/s while descending at 11.9 km/s. What is its speed, in km/s?

Answer 1

down is perpendicular to east (Google "orthogonal")
That means that the resultant is the hypotenuse of the triangle with legs of 23.1 east and 11.9 down.
resultant speed = sqrt(23.1^2 + 11.9^2)

Answer 2

for example:
https://www.physicsclassroom.com/class/vectors/Lesson-1/Component-Addition

or

https://en.wikipedia.org/wiki/Orthogonality