X-9=12
x-9+9=12+9
You need to add 9 to each side of the equation.
x=21
The answer is going to be 5 laps per day because I did 15 laps/3days and divided each side by 3 which the answer is going to be 5/1 but it’s basically 5 since it’s going to be a whole number in this set. Therefore the unit rate of 15 laps in 3 days is going to be 5 laps per day.
I would think you would use 1/3 of a cup but I am not sure.Hope it helps!