Label the radii OB and OC where AB and AC are tangents. OB and OC are 4.5 cm as they are both radii. Then we can see that OBA and OCA are both right triangles with OBA=OCA=90º. OAB=OAC=30º because the side lengths fit the criteria for a 30-60-90 triangle. OAB+OAC=60º
If the painting lies in the middle of the rectangle and half of 20 f is 10 so then half of the painting that is on half of the wall would be only 1.2f if you take 10-1.2= 8.8f your final answer.