Let's assume that "s" is the price for a small candle and "l" the price for a large candle. The first condition says that you can buy 6 small candles and two large for $28, this is mathematically equivalent to:

The second condition says two small and six large and sells for $36. This is equivalent to:

Therefore, we have a system of equations, to solve this system we will first solve for "s" in equation (1), first by