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!