SAT Question edc1b7b7: Systems of Linear Equations answer and explanation with Desmos solution

Adding the second equation to the first equation in the given system of equations yields $\left(2\left(8x\right)-2\left(8x\right)\right)+\left(4\left(7y\right)+4\left(7y\right)\right)=12+12$, or $8\left(7y\right)=24$. Dividing both sides of this equation by $8$ yields $7y=3$. Substituting $3$ for $7y$ in the first equation, $2\left(8x\right)+4\left(7y\right)=12$, yields $2\left(8x\right)+4\left(3\right)=12$, or $2\left(8x\right)+12=12$. Subtracting $12$ from both sides of this equation yields $2\left(8x\right)=0$. Dividing both sides of this equation by $2$ yields $8x=0$. Substituting $0$ for $8x$ and $3$ for $7y$ in the expression $8x+7y$ yields $0+3$, or $3$. Therefore, the value of $8x+7y$ is $3$.