SAT Question c01f4a95: Linear Functions answer and explanation with Desmos solution

It's given that for the linear function $j$, $m$ is a constant and $j\left(12\right)=18$. Substituting $12$ for $x$ and $18$ for $j\left(x\right)$ in the given equation yields $18=m\left(12\right)+144$. Subtracting $144$ from both sides of this equation yields $-126=m\left(12\right)$. Dividing both sides of this equation by $12$ yields $-10.5=m$. Substituting $-10.5$ for $m$ in the given equation, $j\left(x\right)=mx+144$, yields $j\left(x\right)=-10.5x+144$. Substituting $10$ for $x$ in this equation yields $j\left(10\right)=\left(-10.5\right)\left(10\right)+144$, or $j\left(10\right)=39$. Therefore, the value of $j\left(10\right)$ is $39$.