Is $(x_1,x_2) \mapsto \frac{x_1}{a-x_2}+b$ convex?

Question

I encountered the following objective function:

$$D(x_1,x_2) = \frac{x_1}{a-x_2}+b$$

Is it convex? If I use the Hessian to determine its convexity, I have

$$H = \begin{bmatrix} 0 & 0 \\ 1 & \frac{2x_1}{(a-x_2)^3} \\ \end{bmatrix}$$

but the determinant of $H$ is zero, so we can't determine its convexity by using the Hessian value, right? So, how can we do? Thanks for any comment.