GENERAL KNOWLEDGE: Those such as Pilatre de Rozier and the Marquis d'Arlandes?

Questions

Solve it: 3x + 2x?

Solve it: 7y - 2y + 3y?

Solve it: 14a x 3a - 8a ?

The grass on the prairie can be eaten up by 27 cattle in 6 weeks. If there are 23 cattle, the grass can be eaten up in 9 weeks. If there are 21 cattle, how many weeks would it take for the grass to be eaten?

Sue went to the store and bought $5.25 worth of sodas and $2.80 worth of bread. What will be the subtotal of her bill?