15 square feet. If one side is 3 the opposite side has to also be 3. And since 3 + 3 = 6 that means that you have 10 left. If you split 10 in half (5 and 5) and draw out the rectangle you will see that two sides are 5 feet while the other two are 3 feet. The perimeter of that rectangle would be 16 feet and the area would be 15 feet.
You know that 2 sides are 3 feet, if the said baby quilt is a rectangle. Anyway, 2(of the sides) x 3 = 6. So that's 6 of the 16 feet of the perimeter. The other sides added together is 10, and since you have to split 10 in 2, the other sides are 5 feet each. So you have 5+5+3+3=16. Now you take two side that are perpendicular to each other(one 5 feet and another 3 feet) and multiply them. 3x5=15. So the area is 15 feet squared.
