Given:
Area of trapezoid is, A = 243 cm².
The height of the trapezoid is, h = 18 cm.
One parallel side of the trapezoid is, a = 10 cm.
The objective is to find the second parallel side is, b.
The formula to find the area of the trapezoid is,
Now substitute the given values in the above formula to find the second parallel side b.
Hence, the length of the second parallel side is, b is 17 cm.