check the picture below.
so.. notice, the box for the gift is really just a rectangular prism, which is really just 6 rectangles stacked up to each other at the edges.
the front and back are two 5x3 rectangles
the left and right are just two 3x7 rectangles
and the top and bottom are just two 5x7 rectangles
so, just get the area of each, sum them up, and that's the area of the rectangular prism and thus of the box, and that's how many in² she will need.