Đặt \(z=a+bi\) với \(a,b\in R\). Khi đó \(\bar{z}=a-bi\) \(2\bar{z}-i.\bar{z}=2+5i\Leftrightarrow 2(a+bi)-i(a-bi)=2+5i\) \(\Leftrightarrow (2a-b)+(-a+2b)i=2+5i\) \(\Leftrightarrow \left\{\begin{matrix} 2a-b=2\\ -a+2b=5 \end{matrix}\right.\Leftrightarrow \left\{\begin{matrix} a=3\\ b=4 \end{matrix}\right.\) Suy ra \(z=3+4i\Rightarrow \left | z \right |=\sqrt{3^2+4^2}=5\)