I'm not good with the names of each of the panels so I hope I can explain it properly.It may depend on which version of visual studios you have.I'm using VS2017, so I would click and drag the tabs over the bottom panel.While I am dragging a pop up appears that show where you want to align the window.Each panel has one, like the code view panel, where you read/write code and has the tabs of each file like example.cs tab.But the bottom panel, which has the output and error list tabs has its own little pop up when you drag a tab over it.Make sure that you drag and drop the tab(solution explorer) on the pop up of the bottom panel, but in the center of it,That way the solution explorer tab will join the other tabs of the bottom panel instead of making a new window.Hope this helps!