The angles formed will be right angles.
Step-by-step explanation:
A right angle measures 90 degrees. It is formed when two straight lines- one horizontal and one vertical meet at a point. The sides of a poster would consist of a vertical line and a horizontal line.
So when they meet at the corner, they form a right angle. The poster would also be most likely rectangular or square in shape. Square and rectangles have right angles at their corners and so that angles formed will be right angles.