Find the area of the parallelogram, find the area of the triangle, then subtract the triangle's area from the area of the parallelogram.
Area of Parallelogram:
A= base * height
A= 20 in * 10 in
A= 200 in^2
Area of Triangle:
A= 1/2 base * height
A= 1/2 (10 in)(9 in)
A= 1/2 (90)
A= 45 in ^2
Area of Poster Left:
subtract the difference
A= Parallelogram Area - Triangle Area
A= 200 in^2 - 45 in^2
A= 155 in^2
ANSWER: 155 in^2 is the area of the poster board she has left.
Hope this helps! :)