Which property is this equation equal to? (sorry if i worded that weird!) 15 + (5 + 7) = (15 + 5) + 7

What is negative 3/9 times negative 2/8 ?

2x minus y equals 6, x plus y equals negative 3

Awater truck is filling a swimming pool. the equation that represents this relationship is y=19.75x where y is the number of gallons of water in the pool and x is the number of minutes the truck has been filling the pool.