x1+x2=1-3a;
x1x2=2a²-1;
(3x1-x2)(x1-3x2)
=3x1²-9x1x2-x1x2+3x2²
=3x1²-10x1x2+3x2²
=3(x1+x2)²-16x1x2
=3×(1-3a)²-16(2a²-1)
=3+27a²-18a-32a²+16
=-5a²-18a+19=-80;
5a²+18a-99=0;
(5a+33)(a-3)=0
a=3或a=-33/5;
Δ=（3a-1）²-4(2a²-1)=9a²+1-6a-8a²+4=a²-6a+5≥0;
∴(a-1)(a-5)≥0;
∴a≥5或a≤1;
∴a=3舍去；
a=-33/5;
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