I’m stuck on this question 3(-4)-6(-5)/8-6

There are 15 plates and each plate is either 7 or 12 inches diameter. which systems of equations do you use to find the number of 7 inch plates, x and 12 inch plates, y

Acircle has a radius of 5/6 units and is centered at (3.6, 7.8) write the equation of this circle

Jack has a piece of red ribbon that is two times as long as his piece of blue ribbon. he says that he can use two different equation to find out how long his piece of red ribbon is compared to his piece of blue ribbon is. is he correct? explain his reasoning