Create polyhedron for given matrix A and vector b

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$Ax \leq b \iff Ax - b \leq 0 \iff (-A)x + b \geq 0.$

So if you have $(A, b)$ such that $Ax \leq b$ is your polyhedron, then the pair $(A', b)$, where $A'=-A$, is such that $A'x + b \geq 0$ is also your polyhedron.