The correct answer is 50 cm or 500 mm. There are 4 base edges and 4 lateral edges, so just multiply the number for both edges by 4 and add. The question threw a curveball at us by giving us two units of measurement, millimeters and centimeters, but since we know that there are 10 millimeters in a centimeter, we can just divide 55 by 10 to get 5.5.
5.5 x 4 = 22
7 x 4 = 28
28 + 22 = 50
If it helps, you can draw the figure and label the edges. Remember that the lateral edge of a quadrilateral pyramid is the slanted line leading up to the top of the pyramid, and that the base edges are the sides of the quadrilateral on the bottom.
Hope this helps!