Once you know $x$, plug that value into $y = 3\sqrt{x} - 6$ to find the corresponding $y$-value.

Compute $\sqrt{81}$ first, then multiply by 3, and only then subtract 6. Keep the order of operations clear.

In Desmos, type x = 81 on one line and y = 3*sqrt(x) - 6 on another line. This will graph a vertical line and a curve.

Click on the point where the vertical line and the curve intersect. Desmos will display the coordinates $(x,y)$ of that point; read off the $y$-value shown there.

At the intersection, the point $(x,y)$ must satisfy both equations:

• $x = 81$
• $y = 3\sqrt{x} - 6$

From the first equation, we know immediately that the $x$-coordinate of the intersection point is $x=81$.

Now plug $x=81$ into the second equation for $y$:

So we need to evaluate $3\sqrt{81} - 6$.

First find the square root:

• $\sqrt{81} = 9$ because $9^2 = 81$.

Now substitute this into the expression for $y$:

So the value of $y$ at the intersection is $21$.