5. Linear algebraic equation

Solve[a x + b == 0, x]

eq = {q1 + q2 == f,  a q1 == b q2}
qq = {q1, q2}

Solve[eq, 
        qq] /. a + 
          b-> l

Solve[a x^2 + b x + c == 0, x]

% /. {a ->  1, b ->  -2, c -> - 3}