[artty60]

$(a^3-b^3)=(a-b)(a^2+ab+b^2)$

$19x^2=a^2+ab+b^2........(1)$

$(a-b)^2=a^2-2ab+b^2=x^2........(2)$

$(1)-(2)\rightarrow 18x^2=3ab$

$\therefore ab=6x^2.......(3)$

$(2)+(3)\rightarrow a^2-ab+b^2=x^2+6x^2=7x^2$