The trickiest part about this problem is figuring out how to translate the words into math symbols! We know "sum" means to add. We are also talking about "a number" that we don't know yet so we can represent this number with a variable. I'm going to use the variable "x" because that's the most common. So we are talking about the sum of a number and 9, which means adding x and 9, or x+9. Whenever you see the word "is" in a word problem that means equals (=). So our word problem becomes the math problem x+9=-30. At this point all you need to do is subtract 9 from both sides to solve for x using basic algebra.
