What sign to put when finding acceleration?

Question

Physics

The skater, having accelerated to a speed of 6 m/s, began to glide uniformly. After a time of 30 seconds, the speed modulus of a skater moving in a straight line became equal to 3 m/s. Find the skater's acceleration, assuming it to be constant.

The question is, what sign do we put? It is clear that it slows down, therefore the acceleration is negative, but here in this tutorial it is said that it only shows that it is directed opposite to the X axis. And the answer is 0.1m/s^2. But if it is 0.1, on the contrary, has it acquired even greater speed?

Answer 1

Acceleration is not a number, but a vector .
The module (value) of the vector is 0.1 m/s^2.