Solve the real-world situation by using the substitution method. The number of DVDs sold at a store in a month was 940 and the number of DVDs sold decreased by 12 per month. The number of Blu-ray discs sold in the same store in the same month was 522 and the number of Blu-ray discs sold increased by 26 per month. Let d represent the number of discs sold and t represent the time in months.