Answer:
8 cm
Step-by-step explanation:
The area of the shaded portion is:
[tex]a = \pi \times (( {r1)}^{2} - {(r2}^{2})) [/tex]
A=39π cm^2
r1 is the radius of the large circle
r2=5 cm
So,
[tex]39\pi = \pi \times (( {r1)}^{2} - {5}^{2} ) \\ r1 = 8 \: cm[/tex]
