Think of it like this. Do you know what 4 less than a number n is? Say n=5. What's 4 less than 5? That's 1. What's 4 less than, say 7? 3. We can write an expression: n-4.
So, if -13 is 4 less than n, or n-4, we have an equation: n-4 = -13 . Are you familiar with negative numbers? Well, what's 1-2? That's -1. Why? Well, that's equal to 1-(1+1), which is equal to 1-1-1. 1-1=0, so this simplifies to 0-1. Can you get any lower than 0? Yes: using negatives! 0-1 equals 0 + -1. Just like 0+1=1, 0 + -1 = -1.
Anyway, now we know about negative numbers. Just think of -13 as 13 less than 0. So we have n-4 = -13. This could be your expression, if that's all you need.
Now, to solve, we need to find n, so we need to isolate it by adding 4 (the inverse operation) to both sides. n-4+4 = -13+4. n-4+4 equals n, but what's -13 + 4? Well, if you're 13 below 0, but then you add another 4, then...you're 9 below 0, or -9! So, n = -9.
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