Linear functions SAT question

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