You have to solve this system of linear equations:

 a + 2b + 3c = 9
 2a - b + c  = 8
 3a - c = 3

So you can solve it following:

 var eqs = require('eqsolver');
 
 l = [[1, 2, 3], [2, -1, 1], [3, 0, -1]];
 r = [9, 8, 3];

 return eqs(l,r);

 // => {a:2, b:-1, c:3}

It's easy!

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