First find the area of the square (that is half made with the 11cm line). This is 9x11 which is 99. You have the 2 equal triangles to either side left. Because they are equal, you can work out that because it is 18cm - 9cm = 9cm and 9cm / 2 = 4.5cm, meaning that you have to do 4.5cm x 11cm to equal 49.5cm, but because it is a triangle you halve it, making 24.75cm (but because you are adding these together, you end up with 49.5cm anyway). Finally, you add thrm together : 99cm + 49.5cm which equals 148.5cm squared
