How to tell when to use positive or negative symbols when dividing?

Three cylinders have a volume of 2836 cm^3. cylinder a has a height of 900 cm. cylinder b has a height of 225 cm. cylinder c has a height of 100 cm. find the radius of each cylinder. use 3.14 as an approximate for π

When x = 3 and y = 5, by how much does the value of 3x2 – 2y exceed the value of 2x2 – 3y ?

The perimeter of a rectangle is 42 inches. if the width of the rectangle is 6 inches, what is the length