Once you know $x$, replace $x$ in the second equation $x = y + 3$ with that value. What equation in terms of $y$ do you get?

You should now have an equation like $7 = y + 3$. How can you isolate $y$ using inverse operations?

In Desmos, type x = 7 to graph the vertical line that represents the first equation.

In a new line, type x = y + 3 (or equivalently y = x - 3) to graph the second line.

Click or tap on the point where the two lines intersect. Desmos will show the coordinates of this point $(x, y)$.

Look at the $y$-coordinate of the intersection point. That $y$-value is the solution to the system and answers the question.

From the first equation, $x = 7$, so we know the exact value of $x$.

Take $x = 7$ and plug it into the second equation $x = y + 3$.

This gives the equation

To solve $7 = y + 3$ for $y$, you need to get $y$ alone. Subtract $3$ from both sides of the equation:

Evaluate $7 - 3$ to get $4$, so $y = 4$.

Therefore, the value of $y$ is $4$.