-16=x+20
If you do the same thing to both sides of the equals sign, it will stay the same. Eg if you had 2=2, then adding 5 to both sides would make it 7=7 which is still correct.
Using this method, subtracting 20 from both sides will cancel out the 20 on the right and just leave -36 on the left.
-36=x
This is the final solution.