a = (0,4)T is aligned with the Y axis. Its length, 4 units, can be read off the diagram.
Of course, vectors have no fixed location, so vector a can be drawn anywhere. The diagram shows the vectors of length 3 and 4, and with a new vector, h.
The length of vector h is harder to figure out. But not much harder, especially if you know about "3, 4, 5 right triangles." The three vectors can be arranged into a right triangle with h as the hypotenuse. The other sides are 3 and 4, so the length of h is 5.
You probably know the Pythagorean Formula:
(length of hypoteneuse)2 =
(length of first side)2 + (length of second side)2
Using this with a right triangle with sides of 3 and 4:
(length of hypoteneuse)2 = 32 + 42
(length of hypoteneuse)2 = 9 + 16 = 25
(length of hypoteneuse) = 5