Limits Continuity And Differentiability

If \(\mathop {\lim }\limits_{x \to 1} {{\sin (3{x^2} - 4x + 1) - {x^2} + 1} \over {2{x^3} - 7{x^2} + ax + b}} = - 2\), then the value of (a \(-\) b) is equal to ___________.