consider a point on a 2 dimensional graph with imaginary numbers measured on the vertical and real numbers on the horizontal and calculate the distance from the origin (0,0) to that point, (6,2i). The magnitude of the line, or vector, connecting those 2 points is the square root of 62+22 or the square root of 40, or twice the square root of 10, or twice 3.16 or about 6.3
The magnitude or modulus is the length of the hypotenuse of a right triangle with sides 6 and 2.
Another method is take the given complex number, and multiply it by its conjugate, then take the square root. The conjugate is 6-2i. (6-2i)(6+2i)=36+4=40 Take the square root. 401/2=about 6.3