To find the missing dimension you will use the formula for finding the volume of a prism and solve for the missing dimension. In this case 2ft is how deep the pool is and you will solve for the width of the pool.
V = Bh, where B is the area of the base. In this case, use the area of a trapezoid formula.
286 = 1/2h(8 +13)2
286 = 21h
21 21
h = 13.6
The width of the wading pool is approximately 13.6 feet.