To isolate \( r^2 \) in the equation \( 36\pi = 4\pi \cdot r^2 \), you can first divide both sides by \( 4\pi \):
\[ \frac{36\pi}{4\pi} = r^2 \]
The \( \pi \) cancels out, and you are left with:
\[ \frac{36}{4} = r^2 \]
Calculating \( \frac{36}{4} \) gives:
\[ 9 = r^2 \]
To solve for \( r \), you take the square root of both sides:
\[ r = \sqrt{9} \]
Thus:
\[ r = 3 \]
So, the final value of \( r \) is:
\[ r = 3 \]