[tex]\it a^2-b^2=18 \Rightarrow (a-b)(a+b)=18 \Rightarrow (a-b)\cdot\sqrt3=18 \Rightarrow \\ \\ \Rightarrow a-b=\dfrac{^{\sqrt3)}18}{\ \ \sqrt3}=\dfrac{18\sqrt3}{3}=6\sqrt3[/tex]
