$A = 2x^2+6y^2-6xy-9y+2x+7= x^2 - 6xy + 9y^2 +x^2+2x+1-3y^2-9y - \dfrac{27}{4} + \dfrac{51}{4} \\
= (x^2 - 2.x.3y + 9y^2) + (x^2+2x+1) -3 \left ( y^2+3y + \dfrac{9}{4} \right ) + \dfrac{51}{4} \\
= (x^2 - 2.x.3y + 9y^2) + (x^2+2x+1) -3 \left ( y^2+2.y . \dfrac{3}{2} + \dfrac{9}{4} \right ) + \dfrac{51}{4} \\
= (x-3y)^2 + (x+1)^2 -3 \left ( y + \dfrac{3}{2} \right ) ^2 + \dfrac{51}{4}$
Vì $\left\{\begin{matrix}
(x-3y)^2 \geq 0, \ \forall x,y \\
(x+1)^2 \geq 0, \ \forall x \\ \left ( y + \dfrac{3}{2} \right ) ^2 \geq 0, \ \forall y
\end{matrix} \right.$
Do đó $A \geq \dfrac{51}{4}, \ \forall x,y$
Vậy GTNN của $A$ là $A_{min} = \dfrac{51}{4}$