You know that $x=-3$. Put this value into the equation $y = x^2 - 2x$ to express $y$ in terms of numbers only.

When you simplify $(-3)^2 - 2(-3)$, handle the square and the product with negatives separately. What is $(-3)^2$? What is $-2$ times $-3$?

After you find each part of the expression, add them together to get the final value of $y$.

Type y = x^2 - 2x into one expression line and x = -3 into another line. This will graph a parabola and a vertical line.

Tap or click on the point where the vertical line $x=-3$ crosses the parabola. Desmos will display the coordinates of this point; note the $y$-value shown there.

For two equations in a system, an intersection point $(x,y)$ is a pair of values that makes both equations true at the same time. Here, the first equation already tells you the $x$-coordinate: $x=-3$.

The second equation is $y = x^2 - 2x$. Since the intersection must have $x=-3$, replace $x$ with $-3$:

This gives an expression you can simplify to find $y$.

First square $-3$:

Then multiply $-2$ and $-3$:

So the expression for $y$ becomes:

Now add the two results:

So, the $y$-value where the graphs intersect is 15, which corresponds to choice C.