[tex](a - 2b) {}^{2} - 2a(3a - b) = {a}^{2} - 4ab + 4 {b}^{2} - 6 {a}^{2} + 2ab = - 5 {a}^{2} - 2ab + 4 {b}^{2} [/tex]

[tex]\displaystyle (a-2b)^2-2a(3a-b)=a^2-4ab+4b^2-6a^2+2ab=-5a^2-4ab+4b^2+2ab=-5a^2-2ab+4b^2[/tex]