SAT Question 7625073d: Linear Equations In Two Variables answer and explanation with Desmos solution

It’s given that the equation $7g+7b=840$ represents the number of blue tiles, $b$, and the number of green tiles, $g$, an artist needs for an $840$ -square-inch tile project. It’s also given that the artist needs $71$ blue tiles for the project. Substituting $71$ for $b$ in the equation $7g+7b=840$ yields $7g+7(71)=840$, or $7g+497=840$. Subtracting $497$ from both sides of this equation yields $7g=343$. Dividing both sides of this equation by $7$ yields $g=49$. Therefore, the artist needs $49$ green tiles for the project.