tolong jawab please... pake caranya ya

R = 4 cm
r = 2 cm
p (jarak antar pusat) = 4 cm + 2 cm + 2 cm = 8 cm
d (garis singgung persekutuan dalam) = ?

d² = p² - (R + r)²
d² = 8² - (4 +2)²
d² = 64 - 36
d² = 28
d = √28
d = √4×7
d = 2√7