What we have here is a rectangle and two triangles. To find the area of a rectangle we need length and width. To find the area of a triangle we need base length and height. The picture gives all of these.
Rectangle area=3×10=30sq cm.
Triangle heights+width of rectangle=11, so triangle heights=11-3=8cm. Let’s assume the triangles have the same height 8/2=4cm, then area of each triangle is 4×10/2=20sq cm, so total area of the two triangles=40sq cm. so area of hexagon 30+40=70sq cm.
Now let’s suppose the triangles don’t have the same heights, so one triangle has height=3cm and the other 5cm. What is the combined area of the triangles? It’s 3×10/2+5×10/2=8×10/2=40sq cm. Therefore it doesn’t matter what the individual heights of the triangles are, their combined area is always going to be 40sq cm, so the area of the hexagon is always 70sq cm.