Welcome to the forum.
The trick is to find the right way to split up the 7x. In this case 6x and 1x. But how do you know that split? Why not 5x and 2x?
Look at the diagram below. It may help you to make the split but it still needs a bit of trial.
The four elements must be 2x^2, ?1, ?2 and 3. Put them into the boxes.
?1 and ?2 have to add up to 7x.
It is uncertain whether to put 1 or 3 with the 2x. Whichever you choose, the other will then be the other number, 3 or 1.
So try it out with 3 and 1 one way round, and if that fails, swap them around and you're done.
My second diagram shows what happens if you get the 3 and 1 the wrong way round.
Hope that helps with the next one.