The confusion here is the multiplication and addition, let me clear
In the -1 value of the table
X²= -1×-1 (first = - x -=+ and 1×1 is 1 so the answer is 1) you apply this to every other multiplication
For the addition let me add the 4 value of the table
We have 16+(-20)+3
First you multiply the signs +×-=-
Then you have
16-20+3
-4+3
-1 this is the answer
Tip
-×-=+
+×+=+
-×+=-
+×-=-
Then when you have a bigger number being subtracted from a smaller number, for example
16-20
You subtract the small number from big number
20-16
4
Then add a minus sign
-4