Any i don’t really understand this

Comment
The point is how to remove the brackets when a minus sign is on the left hand side. A second problem is how to get all the terms on one side over to the other.

Step One
Remove the brackets on the left side. This is an absolutely necessary step in the rest of your mathematical studies. What was done in this step affects all three terms on the inside of the brackets. Minus signs are real bad things to misuse. Study carefully how to change the signs when multiplying by a minus. 

Rule a minus sign outside the brackets, when multiplied by anything inside the brackets, change the sign to the opposite of what it was.

-20 + 4x + 5x^2 = 20 - 7x^2

Comment
Before doing the last step, look at the answers. Two of them are 40. That is the answer for 20 and -20 when transferred. It's 40 which is a plus. So you should be transferring from left to right.

Step Two 
Transfer from left to right.
-20 + 4x + 5x^2 = 20 - 7x^2      Add 20 to both sides.
4x + 5x^2 = 20 + 20 - 7x^2      Subtract 4x from both sides.
5x^2 = 40 - 7x^2 - 4x                Subtract 5x^2 from both sides
0 = 40 - 7x^2 - 5x^2 - 4x
0 = 40 - 12x^2 - 4x                   Now you have to put them in the right order.

0 = 40 - 4x - 12x^2 <<<<< Answer