a²-b²=1/3
(a-b)(a+b)=1/3
(a+b)/6=1/3
a+b=2
a(3a+5b)-(a-b)(a+2b)=3a²+5ab-a²-ab+2b²
=2a²+4ab+2b²
=2(a+b)²
=2*2²
=8