If you know the components of a vector you can find the vector's magnitude using the Pythagorean Theorem. In the simple case shown here the vector a has a magnitude of 5. But the whole point of this section is that words in Physics don't always mean what you think they mean. Physics is a way of thinking, not a collection of facts. Common words often have specific but unfamiliar definitions in physics.
Vectors have both a magnitude and a direction.
To add vectors, add components. For one-dimensional motion the sign of a vector simply denotes its direction. Detailed definition of terms and equations relating acceleration to velocity and position, etc. Introduction of the graphical technique for studying complicated motion.