To find the base of a trapezoid, you need to know the length of at least one of the bases and the height (or the length of the median). Since you know that PO is 13 (top base) and QR is 18 (median), you can use the following formula to find the length of the bottom base (MN):
Height x ((base1 + base2) / 2) = Area
Where the height is the length of the median (QR) and base1 and base2 are the lengths of the two bases (PO and MN).
So, in this case, you would have:
QR x ((PO + MN) / 2) = Area
18 x ((13 + MN) / 2) = Area
Solving for MN:
MN = (2 * Area) / QR - PO
So you can fill the area of trapezoid and substitute the values in the above formula.