A = pi * r^2
A = 3.14 * 50^2
A = 7,850 square miles (area of the 50 mile broadcast signal)
A = pi * r^2
A = 3.14 * 75^2
A = 17,662.5 = 17,663 square miles (area of the 75 mile relayed signal)
Now we just need to subtract the 50 mile signal from the 75 mile signal, so:
17,663 - 7,850 = 9,813 square miles greater
Hope this helps :)