Calculus

$f_x = 2x - 4 = 0 \implies x = 2$. $f_y = 2y - 6 = 0 \implies y = 3$. $f_{xx} = 2 > 0$ and $f_{xx}f_{yy} - f_{xy}^2 = 4 > 0$. Point is $(2, 3)$, so $a + b = 2 + 3 = 5$.