In finding the area of a rectangle, we are going to use the formula
So All you need to do was substitution.
As you can notice, this is a difference of two squares. You can simply solve it by squaring the first term (2m) then add a negative sign and then square the last term (5n). You can also use the foil method. Both would do.
The area was: 4m²-25n²
If you have questions, just comment down below. I Hope it helps