How can I find a mod with negative number?

It's $3$ because $-11 = 7(-2) + 3$.

Another way to see this is to take $-11$ and keep adding $7$ to it until you get a positive number. This works because, if you're working modulo $7$, then adding $7$ is the same as not changing the number (modulo $7$). So:

$-11 + 7 \equiv -11 \pmod 7$, and $-11 + 7 = -4$. Therefore $-4 \equiv -11 \pmod 7$. Well, we're still negative. Let's do it again:

$-4 + 7 \equiv -11 \pmod 7$, and $-4 + 7 = 3$. Therefore, $3 \equiv -11 \pmod 7$.

Or, equivalently, $-11 \equiv 3 \pmod 7$.

How do we know to use $-2$? Let's recall how it works with positives first.

If you want to evaluate $31 \pmod 7$, you first recognize that $31 = 28 + 3 = 7 \cdot 4 + 3$. Therefore $31 \equiv 3 \pmod 7$. What did we do here? We found the largest multiple of $7$ that's less than or equal to $31$.

Alternatively, with division, you can evaluate $31/7 \approx 4.429$. The largest integer less than or equal to this is $4$. Therefore $31 = 7 \cdot 4 + \text{some number}$, where your goal is to determine what $\text{some number}$ is.

This same exact process applies for negative numbers.

If you want to evaluate $-11 \pmod 7$, you need the largest multiple of $7$ that's less than or equal to $-11$. This is $-14$. And $-14 + 3 = -11$, therefore your answer is $3$.

Alternatively, with division, you can evaluate $-11/7 \approx -1.571$. The largest integer less than or equal to this is $-2$. Therefore $-11 = 7 \cdot (-2) + \text{some number}$, where your goal is to determine what $\text{some number}$ is.