Two of the sides here are unknown. The one on the leftmost side is equal to 9, since on the other side it also has a length of 9 feet. The section on the top right has a length of 30-9-9=12 units. Adding all of these values together now, you get a total perimeter of:
15+c=17.50
First you subtract 15 by 15 because you switch operations from adding to subtracting then you would put C under it
Second subtract 17.50 and 15 and that is 2.50 so C= 2.50
