Let w = the width of the rectangle
Let h = the height of the rectangle
w * h = 70.
The formula for the area of a triangle is what?
What is the height of triangle EFG?
What is the base of triangle EFG?
So what is its area in terms of w and h?
Now can you answer the question?
A good place to start any problem is to name your variables, in this case the height and width of the rectangle, and to write down what you already know numerically about those variables.