Answer
$-6$
Work Step by Step
Since, $Jacobian =\begin{vmatrix} \dfrac{\partial x}{\partial a}&\dfrac{\partial x}{\partial b}\\\dfrac{\partial y}{\partial a}&\dfrac{\partial y}{\partial b}\end{vmatrix}$
Here, we have $\dfrac{\partial x}{\partial a}=2$ and $\dfrac{\partial x}{\partial b}=1$
Also, $\dfrac{\partial y}{\partial a}=4$ and $\dfrac{\partial y}{\partial b}=-1$
Now, $Jacobian =\begin{vmatrix} 2&1\\4&-1\end{vmatrix}=2 \times (-1) -4 \times 1=-2-4=-6$
