Question

Please help! I will mark the answer as Brainliest!

Answer 1

The product of a complex number and its conjugate is always the magnitude of the number:

$z\cdot \overline{z} = (x+yi)(x-yi) = x^2-y^2i^2 = x^2+y^2$

Since in your case $z = x-2y$, you have

$z\cdot \overline{z} = x^2+y^2 = 1+5 = 5$