a semicircle has an area of 76.97 km. Find its radius

The formula for the area of a circle is:  A  =  πr².

A semicircle is half of a circle, so its formula is:  A  =  ½πr².

--->   76.97  =  ½πr²

Multiply by 2:  153.94  = πr²

Now, divide by pi, and find the square root ...