What's y doing? It's going from the lower curve to the upper curve . We then form the Riemann sum of that from x=1 to x=2. It helps to draw the curves. When we switch the order, integrating with respect to x. Now what is x doing? Well, x will be going across the same curves but in terms of x: from to (complete the square). And then we form the Riemann sum of that and let y go from 0 to 1.