What Is Wrong with This Model

Short Answer

What is wrong with this model? $\begin{aligned}\operatorname { MAX } X _ { 1 } + X _ { 2 } - X _ { 3 } & \\\operatorname { ST~X } _ { 1 } & \leq 10 \\X _ { 2 } & \leq 10 \\X _ { 3 } \leq 10 & \\X _ { 1 } - X _ { 2 } & \geq 5 \\X _ { 1 } , X _ { 2 } , X _ { 3 } \geq 0 &\end{aligned}$