I was thinking you could calculate the area of the shape provided above, which you didn't include, and then multiply that by 3/4 (also .75). You can also divide the area by four and multiply the answer by 3, if I am correct. Then I think you can turn the area into a new shape.
EX: say the first area is 8cm. 1/4 of 8 is 2. You know have 1/4 of 8. 1/4 of eight ( 2) multiplied three times is six. So you could use that to make a new shape with an area of six centimeters. I would make a rectangle with the two different sides being 2cm and 3cm. This is not the actual answer, just an example.
If you have the shape above, I can help.
