======================================================
Explanation:
Any vector of the form v = (a,b) will have a length of |v| = sqrt(a^2+b^2).
This is derived from the pythagorean theorem. It's the same as saying the distance from (0,0) to (a,b) is sqrt(a^2+b^2).
In this case, a = 3 and b = -4, which means...
|v| = sqrt(a^2+b^2)
|v| = sqrt(3^2+(-4)^2)
|v| = sqrt(9+16)
|v| = sqrt(25)
|v| = 5
The vector is 5 units long.
Note: we have a 3-4-5 right triangle
