Which set is closed under subtraction

Find the length of the following tangent segments to the circles centered at o and o' whose radii are 5 and 3 respectively and the distance between o and o' is 12. what is the tangent segment of cd?

Terri makes a quilt using three sizes of fabric squares the side lenght of each fabric square is the square root of the area

Jada has a monthly budget for her cell phone bill. last month she spent 120% of her budget, and the bill was 60$. what is jada’s monthly budget