Let me set an example to help solve this, say the poster is 100 inches by 90 inches. the length of the poster is reduced by 12% so subtract 12 from 100 to get you new length of 78 inches. the width is increased by 15% so multiply 90 by 15% to get 13.5 inches and add the to the 90 inches you have to get your new width of 103.5 inches. multiply them together to get your new area then divide the new area by the old area, subtract that number by 1 and multiply it by -1.
