SAT Question 4f1342d6: Systems of Linear Equations answer and explanation with Desmos solution

It's given that in August, the car dealer completed $15$ more than $3$ times the number of sales the car dealer completed in September. Let $x$ represent the number of sales the car dealer completed in September. It follows that $3x+15$ represents the number of sales the car dealer completed in August. It's also given that in August and September, the car dealer completed $363$ sales. It follows that $x+\left(3x+15\right)=363$, or $4x+15=363$. Subtracting $15$ from each side of this equation yields $4x=348$. Dividing each side of this equation by $4$ yields $x=87$. Therefore, the car dealer completed $87$ sales in September.