So to work out 'x' (or whatever the unknown value is) you can just pull the numbers to the otherside to it ends up looking like
x = 123
So if we have 3x + 9 = 861, we can first MINUS 9, because to take 9 off one side, we must do it to the other, so take 9 away from 861
Now 3x = 852
So to get...