He found the modulus from the complex number. That is to say, given a complex number x+iy, whose co-ordinates are (x,y) on the Argand plane, the modulus |x+iy| is equal to the distance from (0,0) and (x,y) ie \sqrt{x^2+y^2} and the same for |a+ib|
*sorry nrlwinner for butting in ;S*