When you draw the vectors, you form a right triangle:

Code:

|\
| \
2.9 | \ 4.55
| \
| \
V----->
B

By the Pythagorean theorem, $\displaystyle 2.9^2 + B^2 = 4.55^2$

For part 2, A - B = A + (-B):

Code:

/|
/ |
A - B / | A
/ |
/ |
*<----V
B

The triangle doesn't change from part (1) and so the magnitudes should all be the same (but not the direction).

You can see that the direction of A - B is south and to the west by a few degrees. Use trigonometry to find the upper angle to determine the exact direction.