SAT Question c303ad23: Nonlinear Equations In One Variable And Systems Of Equations In Two Variables answer and explanation with Desmos solution

Dividing each side of the given equation by $3$ yields $x^2-6x-5=0$. Adding $5$ to each side of this equation yields $x^2-6x=5$. Therefore, if $3x^2-18x-15=0$, the value of $x^2-6x$ is $5$.