Let x be the original radius. Given that a semicircle will have an arc length of half of a circumference and C=2pr, the arc length will be just pr.
Now the difference of arc is then going to be:
p(6+x)-px
p(6+x-x)
6p inches
So the arc will be 6p inches longer. Or if you wish to approximate...
≈18.85 inches
