The product of a complex number and its conjugate is always the magnitude of the number:
[tex] z\cdot \overline{z} = (x+yi)(x-yi) = x^2-y^2i^2 = x^2+y^2 [/tex]
Since in your case [tex] z = x-2y [/tex], you have
[tex] z\cdot \overline{z} = x^2+y^2 = 1+5 = 5 [/tex]
