see the figure below to better understand the problem
Find out the length of the side AB
Applying the Pythagorean Theorem
AB^2=3^2+21^2
AB^2=450
AB=15√2 units
EF=AB
so
EF=15√2 units
Subtract the radius of circle E and circle F
The diameter of a circle that is tangent to both circles E and F is
D=15√2-3-3=(15√2-6) units
To find out the radius, divide the diameter by 2
r=(15√2-6) /2 units
r=7.6 units
therefore
The radius is 7.6 units