Here is the solution:
When the semi-circular sheet is bent into an open conical cup, the radius of the sheet becomes slant height of the cup and circumference of the sheet becomes circumference of the base of the cup.
therefore, l = slant height of the cup = 14 cm.
Let r cm be the radius and h cm the height(depth) of the cup. Then,
circumference of the base of the cup = circumference of the sheet