The area A of a portion of a circle is:
[tex]A=\pi\cdot r^2\cdot\frac{\alpha}{2\pi}[/tex]Where alpha is the angle of the portion of the circle. So, to find the radius, we clear 'r' from the expression above:
[tex]38=\pi\cdot r^2\cdot\frac{1}{2\pi}\cdot\frac{7}{5}\pi[/tex]We can cancel the pi on the left of 'r' with the one on the right (the one that's dividing):
[tex]38=r^2\cdot\frac{7\pi}{2\cdot5}[/tex]So, now we clear 'r':
[tex]\frac{38\cdot2\cdot5}{7\cdot\pi}=r^2[/tex][tex]\sqrt[]{\frac{380}{7\pi}}=r[/tex]So, the answer is:
[tex]r=\sqrt[]{\frac{380}{7\pi}}km[/tex]