Apparently you are talking about a complex plane where the x-axis is the real part and the y-axis is the imaginary part of the complex number.
It's like a vector where you are looking for the length of the vector.
the graph forms a right triangle with the x-axis as the adjacent side and the line parallel to the y-axis as the opposite side.
the hypotenuse is the vector and the length of that is found by the pythagorean formula of a^2 + b^2 = c^2 with a being the real part and b being the imaginary part.
Read the reference. It explains it pretty well.
