To find the answer you need to find the volume and would want to think of the sandbox as a rectangular prism and identify the length, width, and height. For this equation the height would be 2 feet because she wants it to be 2 feet deep. For the length and width both the values would be 7 feet because the base is a square.
Using the equation Volume=Width*Height*Length. Then, plug the values into the equation to get V=7*2*7 which leaves you with V=98 feet^3. This means Jamila would need 98 feet^3 of sand to fill the sandbox.