SAT Question 01682aa5: Linear Equations In Two Variables answer and explanation with Desmos solution

Choice C is correct. It's given that line $r$ is perpendicular to line $p$ in the xy-plane. This means that the slope of line $r$ is the negative reciprocal of the slope of line $p$. If the equation for line $p$ is rewritten in slope-intercept form $y=mx+b$, where $m$ and $b$ are constants, then $m$ is the slope of the line and $\left(0,b\right)$ is its y-intercept. Subtracting $18x$ from both sides of the equation $2y+18x=9$ yields $2y=-18x+9$. Dividing both sides of this equation by $2$ yields $y=-9x+\frac{9}{2}$. It follows that the slope of line $p$ is $-9$. The negative reciprocal of a number is $-1$ divided by the number. Therefore, the negative reciprocal of $-9$ is $\frac{-1}{-9}$, or $\frac{1}{9}$. Thus, the slope of line $r$ is $\frac{1}{9}$.