You start with 900= -100x + 300
you want to get x by itself on a side of the equation
you can do this by adding -100x to both sides of the equation
this would make it look like 900 + 100x = -100x + 100x + 300
if you simplify, you get 900 + 100x = 300
then subtract 900 on both sides to get x by itself
you will end up with 100x = -600
you divide both sides by 100 to get x by itself and you will end up with
x = -6
if you have any questions ask me and ill answer :)