You can start by copying a selected formula (left click for the full expression or drag the mouse cursor over the sub-expression while holding the left button of the mouse) from the problem definition using the "Copy to a new row" action.
OR
You can use the tools panel to create a new row (the first button from the bottom) and copy the expression and paste it into the row you created or just click on the expression or use keyboard shortcuts such as Ctrl + C (command +C) and Ctrl +V (command +C). You can also select parts of an expression using the tools panel or just drag your cursor over the expression you want to partially select.
Also, you can use "Click here to create a new row" to enter your work directly.
HIGHLY RECOMMENDED: Please see this article for more details on problem-solving.