After you set the expressions equal, move all terms to one side so that the equation equals 0. What quadratic equation in $x$ do you get?

Your equation should now look like $x^2 + 8x - 20 = 0$. To factor this, look for two numbers that multiply to $-20$ and add to $8$.

Once you find the two solutions to the quadratic equation, check which of those values appear in the answer choices.

In Desmos, enter the two equations exactly as given:

• y = 20
• y = x^2 + 8x

This will show a horizontal line and a parabola on the same coordinate plane.

Click on each point where the line and the parabola cross; Desmos will display the coordinates of these intersection points. Look at the x-coordinates of these points.

Compare the x-coordinates of the intersection points you see in Desmos with the answer options $-8$, $-5$, $2$, and $10$. The x-value that matches one of these choices is the correct answer.

At the point where the line and the parabola intersect, their y-values are the same.

So set the right sides of the equations equal:

Move all terms to one side so the equation equals 0:

or

This is a quadratic equation in standard form.

To factor $x^2 + 8x - 20$, look for two numbers that:

• multiply to $-20$, and
• add to $8$.

These two numbers will be the constants in the binomials when you factor the quadratic.

The two numbers that work are $10$ and $-2$, since $10 \times (-2) = -20$ and $10 + (-2) = 8$.

So the quadratic factors as

By the zero-product property, either

• $x + 10 = 0$, which gives $x = -10$, or
• $x - 2 = 0$, which gives $x = 2$.

The question asks for one possible value of $x$ and gives the choices $-8$, $-5$, $2$, and $10$. Among these, the only value that actually solves the equation is $x = 2$, so the correct answer is C) $2$.