The magnitude of a vector a is the length of the vector.
We write it ||a||
We can draw a right triangle and apply the pythagoras theorem. The magnitude of a will be the hypotenuse :
||a||² = 3² + 3²
||a||² = 9 + 9
||a||² = 18
||a|| = √18 = √(9*2) = 3√2 ≈ 4.24
So the answeer is 4.24
Good Luck
