Find the rate of change of the distance between the origin and a moving point on the graph of y = x² + 1 if dx/dt = 2 cm/sec.

Here's the solution:

http://calcchat.tdlc.com/solutionart/calc8e/02/f/se02f01013.gif

except I don't understand where they got that first equation D = √(x² + y²)

can somebody please explain..?