To find the total revenue for any object, you simply multiply the price of the object by the number of units sold.
In this case, the revenue for helments would be 3*14, and the revenue for bicycles would be 125*b.
Knowing that total revenue was 917, we can form an equation:
125b + 3*14 = 917
125b + 42 = 917
Subtract both sides by 42:
125b = 875
b = 7
Sawyer sold 7 bicycles