PLANES AND CONVEXITY.
The algebra is on the panels to the right. One objective function, four constraints (the fourth of which subsumes as many as six additional constraints, depending on how you set it up.) Objective function in purple, consumer utility constraint in black, transformation curve in orange, additional orange at the origin discusses possible variations in the dimensions of the consumption Edgeworth Box which is shown in brown.
I have a really colorful one in mind for Monday, illustrating the workings of Walras's Law in an out-of-equilibrium allocation. Perhaps that will offer an opportunity for a tutorial on the Say Aggregation Principle.