X-y=3 work out the value of y-x

We just started the introduction into circles and i have no idea how to do this.

Bob is putting books on a shelf and takes a break after filling 18 1/2 inches on the book shelf. the book shelf is now 2/3 full. how long is the entire bookshelf?

Stacey filled her 1/2 cup of measuring cup seven times to have enough flour for a cake recipe how much flour does the cake recipe calls for