Function \(f(x)\) is defined as \(f(x) = \begin{cases} 3x, & x<1 \\ a-b, & x = 1 \\ 4b-a, & x>1 \end{cases}\) If \(f(x)\) is continuous at \(x = 1\), but discontinuous at \(x = 2\) then the locus of the point \((a, b)\) is a straight line excluding the point where it cuts the line